I too, have asked my dogs if they see color... they give me the deer in the headlights stare... but if I ask them if they prefer the brown dog kibble or the whitish chicken leg quarters or RED deer parts they always chose the red or white over brown...
I have also heard that humans ONLY dream in black and white...
I know for sure that the mid 60's T-bird in a weird dream was less than polished and waxed metallic green with a white interior.
i think it is no more than a guess. If i can dream in full color after being "trained" it is only black and white with shades of gray than animals likely see more color than we realize.
Brent