how i can change a user color only?

[Mythotical]

Ok you misunderstood me, I didn't say edit the usergroup, I said edit the user's profile, but I just took a look so forget what I said.

You have 2 ways you can do this, add a new usergroup with the markup you want and add only that person to it. (Simplest method)

Or give me a few days and I'll see about building a plugin to do what you want.