I suspect this might be temporal dithering. Displays are not capable of displaying all of the colors in the 24-bit color space, so we use temporal dithering to help fill in the gaps. The effect is usually subtle, but I suppose there could be cases where a color falls into a wide gap in the range of colors the display can handle, and results in a harsher dithering effect.
You can disable temporal dithering by setting
Option "RegistryDwords" "DitherAlgo8=3; DitherAlgo6=3"
in xorg.conf's "Device" section. Does that make the issue go away?
Even if the issue does go away when dithering is disabled, it would help for us to see this first hand to make sure that dithering is working as intended. Can you attach an image file containing the actual color? Thanks.