As long as the item (e.g. an enamel cup) is ovenproof, the edding 4200 porcelain brush pens are absolutely fine for enamel.

The pens are designed for painting or decorating on porcelain, glazed ceramic and ovenproof glass. When the design is finished it has to be baked in the oven at 160°C for 25 minutes and it is then dishwasher-safe.