This will ensure that the first 3 nicks get a good contrast between the colors, then the next 3 get the second-best level of contrast ad infinitum.

Yes, that's no problem. But: The first nicks are not necessarily the most active nicks, example:

• A..F join (colors: most contrast)
• A..F idle
• G,H,I join (colors: less contrast)
• G,H,I flame or are otherwise active

To work around that, the nicks would have to be sorted for activeness -- and to do that, we'd need knowledge of all further nicks. And then, we could just use my first algorithm.