So I have only a few rolls of filament and wanted more colours. Searched and found a few thingis and even printed one of the designs.

All used marker pens to colourise the filament. Tried this and on my first attempt, while it did colour the filament, it was very "layery" and of course the felt tip got stuck in the hole and would not come out. Also the pens only inked a side and not very well. There was also colour bleeding because the pens left residue inside the plastic thingis.

I had some old inkjet ink lying about so decided to try it out. Of course only 4 colours CMY and Black.

Printed this thingi - http://www.thingiverse.com/thing:190118 - cut a piece of dense-ish sponge (dish scrubber), pumped a bit of blue ink in and snapped it onto my WHITE filament.

Note to self: Wear gloves next time.

Result was a light blue print as in the image.

Tried the same blue on Yellow filament and the result was a nice green for the fan blades.

That thingi colours the filament on all sides and gives a very even colour.

Must try some other mixes and see.