DIY Tutorial Watercolor Mug Gift Idea - They use nail polish which doesn't seem like it would last very long on a coffee cup. It would be a good idea on something strictly used as home decor though.
Poppytalk: DIY: Colour Blocked Coasters by Penelope and Pip: These are a great gift idea!
high-gloss enamel paint from Benjamin Moore - Tropicana Cabana + Snowfall White. Poppytalk: Mid Century Modern Style Before + After