How about make it so you can edit one specific persons username style?
Do you like this idea?
Some do like the idea, just don't feel we have enough luck to even see a easy way for creating a plugin.
This is non usable to the majority of MyBB forums. Better suited as plugin.
Yaldaram is spot on.
It could be very distracting having hundreds differently colored usernames on your screen, unless of course you only allow staff members to use this feature. Either way, it can be very easily done with making a custom profile field and a small template edit. I don't think it should be core.