To give an even more generalized answer, try this: more dust or looser surface = softer foam. No dust = harder foam.
Another way to state it: Fewer, longer pins = softer foam. Many short pins = harder foam.
This is a general principle, and doesn't always work, but I think it's a good starting point.
Our home track, CRCRC, is sticky clay, hard packed, and watered to keep down the dust. We use pink Tapers and Dremel about 1/2 the tread off to start with. Trinity Bomb One foam (all gray) is a good starting point. On some other near-by tracks, Dirtburners in Detroit where the dirt has more sand in it and gets dustier, for example, we'll use full-tread pink Tapers, and here the slightly softer gray-and-white seems to work better. Longer pins & more dust = softer foam.
Can you go for that, Aaron??
Engineering Without Fear