In the past, I had found great success using a porcelain pen by the French company, Pebeo, that I used to make this mug and this tile coaster. Since Pebeo doesn't make a paint pen in purple, I turned to Sharpie. Sharpie makes oil-based markers that reportedly are permanent on nonporous surfaces. I bought the five-pack of oil-based Sharpies that included purple, as well as blue, green, orange and pink.
Detailed instructions, including baking time, can be found here. Be sure to only use oil-based and not the regular Sharpie markers.
A cellophane bag filled with chocolates and a purple polka-dotted bow finished the Sharpie mug gift.
If you make a Sharpie mug, let me know!