Red ink is, due to its aggressive dye and the interaction with plasticisers, particularly critical and seeps into many plastics, so that the discolouration can no longer be completely removed. This affects all ink manufacturers equally. For demonstrator fountain pens, blue or black inks should preferably be used in order to minimise the risk of permanent discolouration. Material differences between manufacturers like Montblanc and Pelikan are in this respect largely comparable.