Rainbow category plots

I'm not sure there's a way to do this, but does anyone know of a way to make the bars of a one category in a category plot rainbow colored? I would like one group to be black and the other to be multi-colored to match the rest of the images in my panel. Using color as f(z) makes each bar a different color, but I want all of the bars to be the same, just rainbow hued.
That is exactly what I want! One more reason to sign up for beta-testing, I guess.