Other motives for donation or 'upgrades' can be considered. Rainbow, shiny usernames is not one of them.It really depends. Well, the only exception for what you've said here, would be if they are paying for a monthly premium membership where they would receive perks. If a webmaster is going to offer a "premium upgrade", that's the least of incentives they can offer.
I'm not deferring to what you stated. I completely agree, to many coloured will make your board look messy and unorganised. If you find there is an increased demand, you can either choose to either remove this advantage or increase the price of membership.
What are your thoughts?