A few drops of green food colouring were added.
The bread - which tastes nothing like watermelon - is made with charcoal, green tea and strawberry food colouring to create the bright colours of the seeds, rind and flesh.
The card, with a picture of Brussels sprouts on the front, is made from potato starch paper and printed with food colouring instead of ink.
The banned food colouring Sudan 1, a carcinogen, has been discovered in batches of roast chicken wings and chicken burgers on sale at KFC fast food outlets in China.
Mr Buttel said he and his colleagues had bought a new printer and modified it to run on food colouring instead of ink.
Add a tiny amount of pink food colouring paste to the remaining mixture and stir until evenly coloured.