hmm... this is perhaps slightly out of point, but if dichromatism did not compromise their survival as a race it would seem to me that the colour red must not have played an important part in their lives or habitat. In that sense I think the previous post has a point, in that they would simply just fail to notice something as being red, just like how bees see ultraviolet (and depend heavily on it) while we have never had such an evolutionary need so far and thus cannot see outside our present spectrum.