Heyyou27 :
That was generally the case until the release of the Geforce 8 series.
There was a much more obvious gap in color quality between the Nvidia 7 series and the ATI 1k series.
The Geforce 8 series closed the gap and even some reviews by Toms Hardware showed examples of where the color quality on the 8 series was even better than ATI's 1k series.
I can't say because I do not own an 8,9 or 2k or 3k series but the Feb edition of Maximum PC had a multipage review on Nvidia versus ATI where they compared the 8 series to ATI's 2k series and did a double blind test with image experts (photographers, publishers, & artists). 6 out of the 8 when interviewed said that ATI had better color quality in gaming than Nvidia.
Nvidia's color quality increased with their next gen cards but appearently so did ATI's.