Good question, Mindy!
I just recently did some research because I wanted to expand from my wheatens and blue wheatens. I can share what I learned, but there are others here with far more experience that can tell you more about what type of improvement each of these need.
Here is what I learned:
Brown Reds seem to be the most difficult to find.
Silver would probably be next
Buff might be next - I've heard that, even though this is a solid, it is difficult to get even coloring on the Buffs.
Blacks, Blues, Wheatens, and Blue Wheatens are widely available. Whites not quite as easily obtainable.