Last week a guy asked in the brazilian houdini group, in facebook, how to simulate colored smoke. I believe there are lots of hip files with this kind of effect, but while thinking about it, it came to my mind the possibility of using CMYK instead of RGB, since cmyk is more suitable for mixing colored things other than light.
I couldn´t spend more time testing it or improving the file, but it seems to work, so here´s the hip file.
colored_smoke_V002.hip