One way to tackle this is to paint your mugs with an acrylic enamel paint meant for ceramics ... Another way to decorate your mug is with decoupage. All you need is some dishwasher-safe Mod ...
If you can't find the enamel mugs you want in a specific color, don't be afraid to paint them or decorate them to suit your needs. Acrylic paint is great for enamel and metal materials ...