We have an abundance of coloured usernames as it is. The forum most certainly doesn't need more. If anything, we should go back to having some groups without any custom username effects (2013).
This has been suggested a thousand times. Due to it already being a feature on several other competing forums, it will absolutely never be implemented as stated every time this was suggested in the past.
The coding behind this wouldn't be simple, you guys seem to think it can be done in a matter of seconds. If this was ever added I'm sure it would be a Support addition only.
As @"Philly" said though, it won't be added.
I have wanted this for quite some time now, but knowing @'Philly', we'll probably have to pay more money to have access to it if it does indeed get implemented.