I believe we, and other primates, are among the very few mammals that have color vision. It has been theorized that social behavior in primates has favored color vision over time because of the importance of judging moods in others-- when faces flush with anger or pale from depression or illness. This is also thought to be why many primates do not have hair on their faces.

Social behavior can be a very powerful tool in evolution. It could be that because color vision became so important to our ancestors that in the process we gave up a lot of night vision capabilities and therefore gave up on being nocturnal. Another idea might be that night time, itself, was making social/facial communication too difficult for primates and so they evolved to be awake during the day instead. Then, good color vision began to be favored over mono-chrome nightvision do to the advantages of being able to tell good leaves from bad leaves, ripe fruit from unripe fruit, etc. Humans have a fantastic ability differentiating shades of green. that is no accident.