The color of the username is reflective of what your highest star rarity card you have in your collection.
Currently, mine is dark green, indicating that I owe a 6* card.
To have it changed, PM one of the active moderators, stating which card rarity you currently have, and they will confirm it with your account and make the changes.
Click on this link to more info/instructions: RANKING MEMBERSHIP INFO
Also, any other color you see not indicated by a star rarity statement isn't available to anyone.