Anyway at present I am a fan of two types of pen.
Firstly the PaperMate InkJoy Quatro four colour ballpen. I have found two different kinds of this pen, one with pretty normal colours : black, blue, green and red, and one with rather more girlie colours : violet, magenta, indigo, lime green, etc. To be honest, I am not entirely sure what these other colours are, but they are easily distinguished from the normal ones, so they work for me!
This gives me eight colours for mind-mapping and means that I can just take two pens with me and still take notes separating the different headings with different colours. Very convenient.