Technically correct, but there are some issues with this that make it hard on the back end of the Xenforo forum system. Namely, limitations on how many user groups a member can be a part of/how many user groups we can have in total before the system becomes cumbersome and unstable. For me to implement another user group would require me to change a fair number of back-end permissions with the hopes that they don't miraculously break as they did last weekend when everyone except admin lost all permissions. I'm not particularly keen to go through the effort of reinstating everyones user permissions again given the issues I have had in the past - hence more user groups will not be getting implemented purely to change the colour of one username without adequate merit. The issue also arises when I have changed the colour for one username who (sorry for being harsh here) has not been around particularly long - once I have created precedent on this matter it would be unfair to not extend the offer to other members, especially those who have been around for several years. While I would like to do this there is limited capacity for the number of user groups and so I cannot.
I have been considering giving the current user groups different .css styles which would change the appearance of your name/avatar on the left of your posts but have not gotten around to this. If this were implemented then again it would be a user group wide approach - i.e. different groups are separated currently by new user, 5 posts, 25 posts, 50 posts, 100 posts, 500+, moderator and admin - and each of these would have a different .css style. Unfortunately this requires me to a) decide what colour themes/styles I want for each user group and b) actually write the .css and implement it. This is not particularly hard, it just takes time that I do not presently have.
Another idea I have been throwing around is having a few merit based user groups (i.e. best post, best blog, best article, hatbox addicts, longest serving user) with different colours/.css styles. This will be something that may have the capacity for individualization in the future - e.g. if you win best post of the month I could make your text pink/grey/green/blue - but would require testing to see if it's stable, again would require some design on my part and I am unsure how to objectively judge posts given I don't read all of them and the likes system is not used consistently.
I hope this doesn't come off as harsh or lazy. Feel free to let me know your thoughts.