Model paint, nail polish and auto paint are all fine for touch-ups. If your primary concern is the cosmetics then just try to match with whatever type of paint has the closest tint.

You *might* actually have the best luck with nail polish though since they have tons of different colors and sheen-styles (smooth, metallic, glitter, etc.) available for cheap, it's easy to clean-up or redo if you make a mistake and you don't need to buy any brushes or anything.