I was recently working on a node setup the required for some color image data to have some negative values, as I was using the subtract node. As far as I can tell, the values stop being subtracted at 0 though, and just flatten out. I turned clamping off on all of the nodes too. Would a good (but slightly painful) work around be to separate the R, the G and the B values into value data instead of color?
Please site a manual source if possible.