Rabbits are NOT colorblind.
Actually, rabbits do have limited color vision. Rabbits are known to distinguish blue and green wavelengths of light and are likely to possess two kinds of cone photoreceptors in their retina. Humans normally have three kinds of cone photoreceptors, producing well-defined color vision with high resolution known as trichromatic color vision.
Rabbits have Bichromatic Color Vision
Rabbits have bichromatic color vision, and a higher density of rod photoreceptors compared to cone photoreceptors. This adaptation allows rabbits to receive more visual input in low-light conditions, but at a lower resolution. Considering that rabbits have wide-set eyes designed to watch for predators, rather than the bifocal depth perception of humans and other primates, this level of color vision is most likely sufficient for their needs.
Blue and Green Cone Density Varies Across Retinal Area in Rabbits
Interestingly, the green and blue cone photoreceptors are not evenly spread across the retina of a rabbit. The largest portion of the visual field on the retina contains predominantly green cones, while a small lower region of the retina contains almost entirely blue cones. The reason for this adaptation is unknown. One possible explanation is that this is a specialization to spot predators coming from above, in contrast with the blue background of the sky.