RTF is a global modification that replaces all towns in the game.

lol, Khottabych already suggested this on the forums, because of the colors; the fly had purple wings in addition to its red body.

Added 16 minutes later
I sometimes think about switching the locations of the regular upgrade and the alternate upgrade, but I'm holding back on that for now. You can judge by the colors that red is closer to the purple upgrade and looks more harmonious with the original units. You can also look at the design, where I think both units have an equal right to be called an upgrade. And there's also the original concept where the bloody vampire upgrade was the basis of the faction, and the green plague one wasn't so much new troops like the renegades in Haven, but rather a strange, distant province that isn't very popular because of its philosophy, but they can't destroy it and are forced to be friends.