Actually, this question is not as trivial as it first appears. It's getting much harder to find ball point or roller ball refills in colors other than black or blue.
I recall that you used to be able to go to Staples or OfficeMax and get Cross refills in red and green, and your choice of fine, medium or bold. Not any longer. The big box stores really want to simplify stocking, and are trying to drive everyone to use the same boring colors.
Recently, I saw either an advertisement or a post about a roller ball refill that could be filled from bottled ink. There is a much broader choice of ink colors in bottles, so this may be the way to go. Don't know about whether there is a choice of fine, medium and bold.