Merling

Merle is a colour that seems to confuse a lot of people. What is it? Just a dog with random black patches? Nope, far from it! Merle is not a colour in itself, rather it is an add-on gene, that thins out pigment in random areas. When pigment is diluted out like that, it appears different in colour - lighter and paler than usual. A blue merle for instance may often appear to be grey with black patches here and there, but genetically, black is the base colour of the dog!
The two most common merles are blue merle and red merle. Remember what I said about merle being an add-on gene rather than a colour?
-A blue merle dog is a black dog with a merling gene)
-A red merle dog is a liver dog with a merling gene
There are also other possible merle combinations (such as sable merle and fawn merle, which is a type of modified merle found in great danes, and often called fawnequin) but they are rare, and these combinations only accepted in a handful of breeds, if even that. Therefore, I'll focus on the red and blue merle dogs.

Where does the merling appear?
You can never say how much of the coat the merling gene will affect, it is more or less random each time! It might be possible to over time breed for a certain merle pattern, but in most dogs, each one has its own unique number of patches. I have put together a few pictures to give you an idea on how the dog can look, with merle in different amounts.

Different amounts of merle

This dog will represent merles for us today! He is a solid liver. This colour is often called "red" in aussies and other breeds too, but genetically it is nothing but liver, though the shade of the coat can vary quite a bit and appear quite red at times, pale brown at others and sometimes a dark almost lilac colour. Liver is a colour without black pigment. This means that a liver dog can't have any black on it, anywhere. As a result, the nose will be dark brown instead of black, and the eyes will be amber instead of the usual dark brown. The eyerims are fleshy in colour rather than black, though I was lazy here and didn't include that, hehe. This is the case with all liver dogs, regardless of breed.
Note that all terms I'll use here; cryptic, minimal etc, apply to all types of merle colours, not just red ones :)
The dogs with the very least amount of merle are called cryptic merles. In fact, the dog to the left here could be a cryptic merle! Cryptic merle is when the merling appears only in a very, very tiny amount, so little that it's not usually visible when the dog is fully grown! For instance, imagine if this dog was born with merling only on its tail, and later docked. Then you wouldn't be able to tell anymore that it's a merle! However, it's still important that it's registered as a merle and not a liver, because it can often be very dangerous to breed two merle dogs to each other. When two merle dogs are bred you get so-called double-merles. Double-merles are often deaf and/or blind and sometimes born with deformed eyes, or no eyes at all! A few are even stillborn. Therefore, most breeders will never breed two merles to each other, and it's important that dogs are registered with their correct colours, so the breeders can match up their breeding stock right.

This dog has a bit more merling, but it's still a bit less than usual. In this dog, it's easy to see that liver is actually the base colour, not the paler areas as a lot of people think. Dogs that have this little merle are often called minimal merles. In many show dogs, a more even merling than this is preferred.

This is the most common (and in the show ring, preferred!) - the dog has roughly 50 % of each colour. You can never say which areas will be dark and which will be light, there are no "rules". However of course it's pretty unlikely that the dog has completely pale front and dark hindquarters ;)
Merle patches - in all types of merles - can be small, medium, large, very large... Or a combination. Some dogs have large patches of colour, perhaps even a whole leg or more as the base colour, while others have only smaller spots all over. Many dogs will have a mixture of different sized patches. This is all normal. 

Remember the dog with minimal merling? In a way, this dog appears the same way, only opposite colours, but of course, this is a dog with very heavy merling! It is good to note that show dogs are very rarely merled like this, most will have a nice 50 % distribution. If we look at the aussie breed, it is often patched with white, which will make the merling look less extreme. Solid merle dogs are pretty rare, most have white, but the genes aren't tied to each other in any way, so it's fully possible to get merles without white areas too, like in for instance the beauceron breed.
This is what a double merle will often look like. This might requite a little bit more explaining than what I already said in the beginning.
Merle is a dominant gene. There are two type of genes -  recessive and dominant. When a gene is dominant, it means that it needs to come only from one parents to show up in the puppies. If a gene is recessive, it means that you must get it from both parents, or it won't show. You can still carry the trait, though, it just won't peak through without being there "twice". Blue eyes in humans are a good example of recessive genes. Two brown-eyed people can carry the genes for blue eyes and give birth to children with bright blue eyes instead of brown. But blue-eyed parents can't carry the dominant brown eyes (since they already used up their "two gene spots" to get blue eyes!) so they will only have blue-eyed children.
So, when you want merle puppies, all you need is one merle parent, not two. If you cross a merle with another colour, nothing bad will happen. But if you cross two merles, you're in for trouble. If the puppies are born without pigment in the inner ear or around the eyes, the dog will most likely be deaf and/or blind. This has to do with how the fetus developes, but the aim of this tutorial wasn't to go through advanced canine genetics, so we'll stop there :)
So keep in mind, while double merles can be very pretty dogs and deserve as good homes as much any other dogs, it is normally not a good idea as a breeder  to breed two merle dogs to each other, because chances are the puppies will not be normal if you do. And there is no point in breeding potentially ill/deformed puppies, especially when there are already so many homeless dogs looking for a home!

Shades

As if all the millions of possible patch combinations weren't enough, the shades of the coats can vary! If we switch over the blue merles (gotta give the blueys some attention too, ne?) you can get dogs where the merling appears blue, grey, almost brown and so on. In some breed standard they ask for a certain merle shade, while others aren't so picky. Most of the time an even shade is preferred, but there are also dogs that appear to have several shades in their coats. Here are a few examples of different merling colours, just give to give an idea of the range:

Note though that these pictures aren't meant to be exact examples of the real life colours, they just give an idea of the spectrum :) Likewise in red merles you will find dogs that are dark, light, almost cream, almost grey and so. The possibilities are almost endless!

Nose and eyes

We have already decided that the lighter areas on the merled dogs are diluted pigment. Sometimes the merling will affect the areas around the eyes and the nose. If the dog lacks pigment on certain areas of the nose, you will get flesh markings. That is, pink spots or patches. These are not always a fault but they are not really liked either - lack of pigment means that the skin won't be as protected against the sun, which of course isn't good for an active working dog. Albino people often have problems being out in direct sunlight, since they don't have any pigment, and it's the same thing with all other animals!
If the pigment is lacking around the eye/-s the dog can get one or two blue eyes. In some breeds, like the australian shepherd, the eye can even be half-brown (or amber, as the case may be) and half-blue. Blue eyes is just like flesh nose spots - lack of pigment. The eyes are normally light blue but can also take on a greyish shade. The dog can't have one blue eye and one grey - it always one of those colours or neither. Blue eyes are never really a fault in merle dogs, but often very disliked in other colours. Most of the time they won't even appear (I bet you've never seen a golden retriever with a blue eye!) but there are exceptions - siberian huskies can often have blue eyes and I've seen a blue roan english cocker with a bright blue eye. 

Common mistakes

I will now go through a few common errors when it comes to merle. I'm not pointing fingers here, just giving a heads up :)
1) Red merles with black patches and...
2) Red merles with black noses and dark brown eyes

There are several things that don't work here. First off - a red merle is a liver dog with a merling gene. Theoretically a dog can be a normal red (ie irish setter red) with merling but I can't think of any breed where that is allowed or happens. So the nose should be brown and the eyes amber in the dog above. The second problem is the colour of the patches. It's easy to think that a merle dog is a dog brown or grey dog with black patches, but by now you know that this isn't the case :D Since a red merle is a liver dog with areas of diluted pigment, it should be a combination of liver and the pale almost lilac colour that is often called isabella in dogs. It should be noted, though, that the pale areas of red merles are called all sort of things, such as cream, umber, cinnamon... but are still the same thing. On to the next example!
3) Patches that are a darker version of the main colour, and...
4) Blue noses on blue merles

As with the "red merle" above, there are a few mistakes on this dog, quite similar ones really. If you've done your homework, you might be aware that many blue dogs (such as great danes, italian greyhounds and neapolitan mastiffs), just like livers, don't have black pigment. Their eyes are amber and the noses are dark grey, sometimes almost appearing black, sometimes being much paler. It's very easy to being fooled to believing that this is the case with the blue merle dog, but nope! Remember, a blue merle dog is genetically black, it just has a merle gene. Therefore, the nose should be black :) I should perhaps mention that the pattern above IS genetically possible a - diluted black dog is blue and add merle to that - but it is extremely, extremely rare, and I can't think of a widely recognized breed of dog where the pattern is allowed!
The second thing that is off here is the colour of the patches. If you don't know how the merle gene works it can be tricky to figure out what colour the patches should be, and I've found a lot of otherwise very pretty drawings, but where the patches have sadly been coloured wrong. In a blue merle, the patches should be black, since black is the colour "under the merling gene" :)
5) White areas with merling

Ack, this brings pain to my eyes, LOL!
First off, yet another genetic explanation, bear with me, this is the last one on this page, promise!
White in dogs (or any animal) is not a colour - it lack of pigment all together - remember, albinos are white! White can occur in different patterns, they are called irish spotting, piebald, extreme white piebald etc, but don't worry about that for now. All you need to know right now is that white like merle is an add-on colour, when it appears like this. Take one of our merle dogs from the table in the beginning of the page, and imaging that you dip his paws, chest and face in a bucket of white paint. See what I mean? He appears to have white markings, but it still blue merle (or black, if we wanna go all the way) deep down. So how can the dog above have black (or blue) patches on his white?! Nope, that won't work, I'm afraid! Now, of course you can have a dog that is patched and then the merle can appear to be patched on white. The dog can also have ticking (small spots of colour). But this is obviously not the case with the dog above - it's not white with patches, it has a symmetrical distribution of white. Therefore, the black areas on white are off. Away with them!

But... but... But what about these, then?!
Okay, now I've ruined several nice patterns above and whined about them not being realistic, haven't I? Well, I better give you something in return then! Here are - just for kicks - some different merle dogs that come in fully realistic patterns, yet look almost like they shouldn't! Be aware though that I doubt any show judges would place dogs coloured like this, LOL! (I use my aussie base but the colour itself might fit better on another breed etc)

Blue merle with irish white spotting and ticking

Blue merle with tan markings, irish white spotting and roan

Red merle piebald

Red merle with irish white spotting

Okay, now we have a basic idea on where and how merling appears! I've gone through everything I could think of. I hope you found this helpful :)

Blue merle/white Welsh corgi cardigan/shetland sheepdog