I have heard people use a red Sharpie on FE Starscream's head crest and missiles and Powerizer Optimus Prime's forearms and chest. My probably dumb questions before I try it: Is there any way that I could screw it up? Do you have to do multiple coats?
For Starscream's crest I did one coat, but it gradually came off after normal handling of the figure. It now sort of looks faded red, which is great considering that's how it looks in the show. I took it out of the head (by disassemling the head) to make sure I didn't get any Sharpie in places that didn't need it.