Well, it just seemed like these are the only two pigments commonly mentioned when it comes to balls. There are amelanistic and axanthic balls, but like Tim Bailey said in another thread that there is no mention of anery balls. I just don't ever see anyone mentioning erythriphore, or anyone working with selectively breeding for high red balls when that is usually a common thing in species that have erythrispore as an option to work with.