A geometric shape would be used to indicate what player you are currently facing. This option could be toggled in the options menu. E.g.
Orange / Blue should be one safe color combination. Dark blue + White / Orange + Black to be on the safe side. So even if a person has only monochrome vision, there is still a contrast between a dark grey+white, and a light grey+black color scheme.
This is the best option in my opinion. Especially combined with another setting "Preferred Enemy Color", just like there is a "Preferred Player Color" option now.
What, wait? There's a "Preferred Player Colour" option now? Where did you find that? Also, if you just use the colours white and black, that would be visible for everybody, wouldn't it?
Oh, right. I'm using your mod. Maybe that's relevant information on this topic. Should be in PA by default though, because it's awesome! Visible, yes. Esthetically pleasing, no. And since esthetics (and apparently also visibility) are personal, what better option than to let each person set preferred friendly and enemy colors personally?