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That most mammals are color blind is nothing more than a myth. A few select are, like the marine mammals, but the vast majority are dichromats with red/blue color receptors.

The last common ancestor was in fact a trichromat, having one red and two different blue receptors. When mammals diverged further, monotremes lost one of the blue receptors while other mammals lost the other.

What did reevolve in groups such as marsupials and apes was the green color receptor, which was present in older ancestors, but had been lost in the first mammals.

source: http://rstb.royalsocietypublishing.org/content/royptb/364/15...