How can you add color in

[cellarius]

Now, my guess would have been that he wants to color the username in the already existing "Welcome, username" message at the top of the header, not just adding it once againg a few pixels below.

Which would be done by editing the header template, changing bbuserinfo.username to bbuserinfo.musername in the following line of code (which will preserve the color added to the username by usergroup or individual user settings):

<li class="welcomelink">{vb:rawphrase welcome_x_link_y, {vb:raw bbuserinfo.username}, {vb:link member, {vb:raw bbuserinfo}}}</li>

Or, alternatively, edit the phrase welcome_x_link_y (html should be possible there).