If it's safe, still requires someone to add new color to each affected portrait. I think everyone uses a unique palette which adds some sanity.
(suggestions on recolor? Fray?)
You can use this on top of your patched rom to apply Fray's Olman's + (partial) un-black fix.
Well, actually, I had already thought of a common color for all portraits. In fact, for the portraits I tested, the four colors to have the double black problem are: 0F 30 27 0F (see image below), so the common color must be halfway between # 27 and # 0F and not even a color who is too dark because otherwise it would go unnoticed. So, taking the nes palette as a reference, we have the colors # 17 and # 16. I would choose # 17 because it is a darker shade than # 27 and also because, if we have to choose a common color, Rossetti has blonde hair and mustache, so taking him as a reference, the "dark blonde" (# 17) for the mustache suits him better, in my opinion.