I've been using the nouveau driver for a long time too, and colors are fine for me. What I would do if I were you is:
1) Install the proprietary driver (a pain I know), and try to reproduce, if the colors are good there it likely is a nouveau bug and not your hardware
2) Install Fedora 16 or 17 (you don't need to switch distros just clear 4G of hd space, do a livecd to harddisk install, and fully update it)
3) If the bug is still there with a fully up2date Fedora file a bug against xorg-x11-drv-nouveau in Fedora's bugzilla. Then hopefully Ben Skeggs will take a look and eventually fix it (he may first need some help from you to gather various info).
Note this is not meant as a shameless plug for Fedora, I just don't know any other distro which maintains nouveau as actively as Fedora does.