<<these simple errors should be corrected not justified.>>

First, I think I misunderstood your complaint. I thought you were talking about a color change that showed up on one panel, not a color change for an entire issue. I wrote my response with that idea in mind, but my basic thinking will still be the same here...

My guess is that either the costume was colored incorrectly and the editors didn't have the time or inclination to fix it, or it could have been an editorial decision to try a different color. The fact that the cover shows the correct ( original ) colors is a very compelling argument, but not conclusive. Covers can be produced well in advance of the coloring of interiors and an editorial change could have been made in the time between completion of cover and interior. Also, there are numerous instances where cover colors do not perfectly reflect interior colors. Bottom line is that I don't know why it is different, but it is different, and I feel that keeping it faithful to the original printing is the better choice when faced with discrepancies.
I agree that it is odd ( and frankly, a bad idea ) that a costume color changed inexplicably between issues. I'm not justifying the original change, I'm justifying why we would have kept that change.