Minimap color mods require changing the gamedata file. In practice this means that such a mod can only be used in multiplayer if the host selects it (and then it will apply to everyone). In singleplayer you are always the host, so that is no issue, but you basically would not be able to be able to use it in multiplayer.

An alternative is to select "friend or foe colors" in the options in game. That will make all enemies red, all neutrals yellow, and yourself blue. I don't know if that would help.

What do you mean "would not be able to use in multiplayer"? As long as he hosts the game it will be no problem at all.

Yes. But I'm not sure how easy it is to fill a modded lobby. People might be sceptical abbout playing with some data mod active that they don't know about.

From my experience, many people don't even notice that it's modded. They just join whatever lobby and long after starting the game, are surprised to find out that it's modded.