Been planning on these for a while. I have one of each color, except for the white varieties.
A white Collie is basically mostly white with a tri, sable, or merle head, usually with patches of the same color on their body.
My dog Treader isn't a Collie (he's a Papillon/Sheltie mix), but as an example in Collie and Sheltie terms his coloring would be called "tri-headed white".
I plan on drawing some smooth Collies like this, too. I may or may not get that done tonight.