Welcome to YorkieTalk.
I'm partial to boys, but you will find plenty of people who like both boys and girls.
Yorkies can be difficult to potty train. You may be lucky and have no problems at all. Definitely requires effort on the human's part to give the dog ample opportunity to go in the right place. Praising for correct potties helps a lot.
Regarding color, there are no guarantees if you bring home a pup. The best indication of color is to look at the parents and a couple of generations back. My boys are brothers from different litters and they are different in coloring.
The most important part of looking for a pup is to research reputable breeders. Make sure you get a good health guarantee.
Good luck in your search.