@Ryan-Feng I believe this can be done with a change in the theme templates for your site. We do send over the group label, and that should contain the colour hex value, which you can use as a style attribute for the username itself.
Theme colors / Theme options
To make nodebb more accessible for people with no knowledge of css it would be cool if themes could have options to customize colors or at the very least have a few preset theme colors.
selecting one of the options would change the whole color scheme of the theme, kinda like bootswatches for vanilla but added by theme makers.
vanilla and lavander are both kinda preset up for this with the use of variables.less
if you could add a way to select 1 of X amount of variable files you achieve this idea.
this would allow people to get more and easier mileage out of each theme, and help designers trying to make pay themes
this would be the simplest approach i could think of with maybe the long term goal of greater theme customizing kinda like wordpress http://codex.wordpress.org/Appearance_Customize_Screen