If you want to use a fountain pen and paint watercolour washes over the top you have to purchase special ink for your pen as standard fountain pen is water-soluble. The good news is there are a number of water-based permanent inks that are suitable for use in fountain pens and that become permanent once dried so that you can paint over the top.
I have found that De Atramentis works best in my pens. I have had a number of pens that were not flowing smoothly with Noodlers ink but as soon as I switched to DeA they started to work like a dream. De Atramentis Document Ink comes in many colours as you can see in the above photo… and I don’t even have the full range!
Note: DO NOT put other permanent inks (such as Indian inks) in your fountain pen as they will ruin them. Only put ink into your fountain pen that says ‘suitable for fountain pens’ and even then, putting any pigmented ink into your fountain pen is a risk – make sure you use and clean your pen regularly!