I heard of someone who tested a small amount of bisulphite-MDP2P addition product, and it turned dark red-purple, sort of like MDEA on the colourchart...and sassafras changed dark, dark purple (not as red as the MDP-2P).
It seems that the methylenedioxy group tends to give the purple/red colour, and the rest of the (modified) amphetamine structure gives the black somehow through the amine moiety (yes, a bio word, but I like it). MDEA, because the ethyl group "protects" the amine, gives more of a colour like a non-amphetamine structure would.
Just my little theory, based on nothing but a friend's observations...