Citation:

Refer to this helpful figure I've provided: http://img9.imageshack.us/img9/2208/pokegate.jpg
It's also attached, since Facebook seems to automatically do that now. Cool.

Venonat and Venomoth are both purple, yes, but Butterfree is (1) also purple, and (2) the same purple as Venonat. Venomoth is lilac, not violet. Your argument supports the hypothesis. As for the discrepancy between Caterpie and Venomoth--that thing just came out of metamorphosis.

Furthermore: Butterfree shares Venonat's feet, hands, mouth, eyes, and antennae. This is extremely notable considering the fact that Venonat is nothing more than a purple puffball with said features.
Didn't it ever occur to you that Butterfree looks kind of... nasty?
Venomoth, on the other hand, has colored wings that shaped like those of a swallowtail butterfly, the same head structure as Metapod, and the same eyes as Caterpie/Metapod.

I rest my case, sir. Venomoth looks nothing like Venonat, while Butterfree DOES---and vice versa.

The theory, therefore:
Game Freak made a programming mistake when coding Red/Blue, and the sprites got switched. They simply covered up said mistake for convenience, and perhaps recolored all official art to reflect the change in Pokemon canon.

So. Mind = blown ?