With the amount of people who lurk on IRC, I would think that you would want a uniform color for join / part messages, and only colorise what people are actually saying.

Have a set number of colors, then as you need a color (someone that doesn't have a color assigned says something) grab the color that has the longest time since it was last used.

Rather than trying to computer generate a list of colors, picking the colors by hand might be a better way to make sure that all the colors are visually distinct.