... I'm hard pressed to believe that there is an advantage for colorblindness that would have been selected for in the earliest mammals.
There didn't have to be an advantage for partial colorblindness (they were never totally colorblind), there just doesn't have to be any penalty for the trait to be lost. Same with the inability of some mammals to synthesize vitamin C, no particular advantage to losing it, but with a vitamin C rich diet there was no penalty either and so it could get lost over time. Color vision only works in bright light. Mammals spent a lot of their early evolutionary history as nocturnal creatures, and so could lose this trait without penalty. In fact it appears there were multiple function S cone loss events in the mammalian line, not just one (genomics gives us powerful insights into this today). The article does point out though that "the fact that these gene mutations have spread throughout the populations allows the possibility that the loss of S cones may in some way enhance visual fitness". It is entirely possible that processing of images in dim light could be better optimized through evolution with the loss of the unneeded bright-light color vision baggage.