Depends on what I'm using it for -- basically whatever is the primary meat in the meal is what I'd expect to be in the gravy. So breakfast would have white gravy (sausage & biscuits, breakfast burrito, etc), while a lunch or supper (or dinner) of beef, turkey, etc. would have a brown gravy.
And for those folks who insist on calling the tomato-based sauces that dress their pasta "gravy" -- nope, they are sauce, no matter what your grandma said.
Fish also gets lovely sauces, but no tomatoes or gravy at my table. ;-)