well, I don't think its any specific formula as much as what colors you use... some colors, when you apply the transparency, can change to something else, and you never really know what color you might get...
lol... like this last one, it was all mostly blue and gray and metallic looking, I applied the transparency, and it turned bronze-ish colored... which turned out well this time, but I've ones that turned sickly green or other really nasty colors!