Before the entire mission stuff with BF and GF, GF bought a lot of nail polish and one night while the two were hanging out, GF made BF her test dummy and made him try out some of her nail polish.

This is why his nails are painted cyan like his hair to this day. Because he keeps finding the nail polish that's exactly like the ones GF bought on that day and keeps repainting his nails whenever they start chipping off.

And now he likes his nails being painted but ONLY if it's the exact color GF used on him on that day.