Dogs essentially come in two colours. All others are variations of these two.
Black is the basic dominant colour in dogs. A black dog will have a black nose and dark brown
eyes (unless other modifiers are at work). The coat is an even colour all over, though in some dogs it can get sunburned and get a brown tint.
Red is the basic recessive colour. It is like a black dog but the dog has not a single black hair in its entire coat. If it has ever so little black, liver, blue or isabella tipping anywhere in its fur, it falls into the black category! The nose and eyes are as for a black dog but it is not uncommon with a fleshy spot on the nose (often called a dudley).
Black can also be recessive in some breeds though this is rare overall in dogs. Normally, black is the default colour and can be overridden by red only if the dog is homozygous for red. A recessive black dog is instead basically a red dog that can't produce red, leaving only the black in the coat.
When speaking of dog colours, it is good to know about these colours.
'Black' pigment in dogs is called eumelanin.
'Red' is instead known as phaeomelanin.
Black/red or eumelaning/phaeomelanin as they will be described on the other pages are can be combined in several patterns and on top of that they can be affected independently by different colour modifiers. You can learn more about these on the other pages.