I have some data I want to represent on a choropleth. It's a divergent data vector, but the data has an additional dimension that is a 2-value factor. Some states are "yes", some are "no". I'd like to simultaneously use two diverging color schemes for the data vector, one scheme for "yes" states and one for "no". The trick is to have all four extremes (yes/min, yes/max, no/min, no/max) be immediately distinct, particularly a four-way contrast that would be visible to the color blind. What choices would be optimal for my four "extreme" colors?
I think I need to explain what "diverging" means. A diverging color scheme is one with a "center". Values can be lower or higher than the center, but what is meaningful is specifically the relationship to the center. Thus, "low to high" is the wrong sort of color scheme to use.
Note: accessibility for color-blindness is preferred.