I want to add some color to my unit cards to help me differentiate on sight my different units, but the glossy paper/coating on the cards has me apprehensive about smearing all over the card. What would everyone recommend? Sharpie, Paint pen, ball-point pen, acrylic paint, highlighter, dry-erase marker, other? I am thinking of the top header and towards the bottom.

I keep my unit cards in protective sleeves. So I would probably just put a colored dot sticker on the protective sleeve.

Scan the cards, photoshop them how you want them, print.

I think the protective sleeves with different colors or dots is the good easy solution.