Oh so you just want the one category to be a certain color instead of the color of the forum discription thing? That's a simple fix. Go to admin panel then go to modify the style that you want to change te cat color. Then look for the spot that says
Category Strip Background Color: #something
Change the hex number to a different color that you want it to be. If it's not that I' confused on what your asking and I'll leave it to tony or ffdcsite