If you've been doing GPU renders with clown masks, you may have noticed that Lots of the materials get the same mask for some reason. I found a little bandaid for it, until they fix it in KS10.
Head into the material graph. Create a diffuse material. Create a color adjust. Plug that into the opacity. Diffuse goes into your Material node as a label.
Now make sure your render setting has "include label" in clown mask.
That's it. You've added a completely blank label, and that overwrites however the clown mask is choosing it's color.