Ticking

You've probably noticed that not all white spotted dogs have a clear white coat. When there are spots of colours in it, it's called ticking or roaning:
Ticking, small spots of pigment on a white background, is dominant to clear white. Exactly which gene causes this is not yet known. Many ticked dogs have roan areas (parts where there is more colour than white), though roan appears to be caused by a seperate gene. Ticking is not visible at birth, but developed with age (though it is visible in the skin even in newborns). It seems to often appear heavier on the muzzle and legs, and to a lesser extent on the tail. Ticking can vary from just a few spots to almost roan all over the dog.
There appears to be a type of ticking that does not begin to develop until the dog is several years old, which might be seperate from the regular ticking.
Larger, round spots, as seen in dalmatians, might be a form of modified ticking. Like ticked dogs, their spots are not visible in the coat at birth, though they can be seen in the skin. 
There has been discussion if roan (heavy intermingling of coloured and white hairs) is indeed its own pattern, or perhaps just heavy ticking. Here I will treat it as a form of ticking. Roan can also be called for instance mottle.

A chinese crested with ticked skin

A blue roan australian cattle dog

Australian shepherd with a ticked muzzle