I think that you will need to check with your gravel sources and see what they have. It can vary widely depending on the source material. I have done something where we used two different colors of gravel to denote parking and pedestrian ways, and while they were two colors, I am not sure how much contrast they provided – would have to be pretty dramatic. Might be kind of hard to provide that contrast with texture, as you are talking about a slight difference in the grade of materials most likely (1/2″ vs. 3/4″). You might be able to get the contrast using a hardscape type material it would seem.