you could probabaly add a bunch of different colors and add some like alchohol or somethin like that

 

Ive tried it with red Gel pens and the alchohol did not work on extracting the ink

 

I cut open a pilot gel pen tube and blow the red ink ink a white out pen then shake it very well. This is pretty much the only way to use gel pens for ink. Nthn but a dye