Actually, it wouldn't be that hard to allow users to set their own colors....

(add a field to the user table with the HTML color code then have the select from users table statement include that color field. Then just apply that color to the user's text).
At the same time, this would slow down the board exponentially as more users logged in since you'd have an extra x^2 db calls per user (at least I think that's the math) logged into the board. The way around that would probably be to store the color field in the table that stores the logged in users. (haven't looked at phpBB structure in a while, but I seem to remember one)
So basically, what sperry said.