I get the idea that any plan HIM might have to corrupt individuals use ways to get them to commit a sinful act (mostly) of their own accord. Look at hoe easily Mimi and Minnie acted against Junior in the pages following the ones I posted, and you'll see that HIM never commanded them to do anything (except when Mimi started defending Jeff and Junior). But he certainly never told Minnie to do anything. He just put in a little nudge and let her emotions shape her actions. I think that's the way HIM got Mimi into such a frame of being that the HER essence would bond with her more completely.
And given Minnie's downward spiral, I doubt it would take very much for her to do the same -- and she still has some command over Junior's Nergal, last I remember, so gaining Minnie's cooperation would gain HIM two sets of extra hands.
If Minnie did change colour during the corruption process, I fully expect her to follow HIM's colour scheme: that's the source of the power.
On the subject of the gift, I think it's either from HIM and will be something likely to push Minnie further over the edge, or from someone who actually cares about Minnie's well-being and is likely something that person sent after Mimi's attempt to take Horror's Hand. Assuming he can write Nergalese, it could even be from Dark Danny, who saw the whole thing.