People do IV MDPV, but it's got a reputation for making a mess of ones life, just like heavy IV use of any strong stimulant. The reason you were okay with a larger dose is likely due to a degree of stimulant tolerance (all stims are cross-tolerant to some extent), since you're comparing it to other stimulants (which you presumably have a fair amount of experience with, and hence probably tolerance to).
As for differences in that picture, all of those samples are filthy, particularly the top one. Clean MDPV is white. There are legends of one early batch of tan MDPV that was amazing and qualitatively different from current batches (see the discussion on it in ADD for the debate about whether it was an active impurity or something other than MDPV. Nobody seems to have any left to get analyzed. Nobody's even reagent tested it as a sanity check, afaik). But, in most cases, tan MDPV is just dirty MDPV that feels just like every other batch of MDPV, only with more toxins.
