Well as this thread started as MC6847 (CoCo),
I just ran a side-by-side test of MESS SVN Coco3 vs a real Coco3. Monitor on PC is SyncMaster 204B and on Coco3 a Magnavox Professional RGB 8CM873. MESS is set for a raster effect so the screens look the same as the Magnavox.
Expecting identical colors from these two monitors may be asking too much, but the CLS CoCo colors are pretty darn close with proper adjustment of the Magnavox monitor brightness and contrast.
The most noticeable difference is CLS4 "red" which is quite red on MESS but has some orange content on the Coco3. CLS2 "yellow" seems to have a slight greenish tint on the Coco3 compared to MESS.
The other colors are essentially identical. The MESS colors seem very slightly more saturated but that is probably monitor differences.