Gerald Jacobs, Gary Williams, Hugh Cahill, and Jeremy Nathans genetically engineered mice that have L-type cones. These mice could do a behavioral task that required them to distinguish two wavelengths of light that control mice couldn’t distinguish. This is amazing because it means that a brain architecture evolved to receive visual input from 2 types of cones can also usefully process information from 3 types of cones with no additional evolution.
Mice and most other mammals are dichromats, meaning that they have only 2 kinds of color receptors, not 3, like humans do. Mice don’t have L-type (long wavelength sensitive; most sensitive to red) cones; this is similar to humans with red-green color blindness. This was the receptor type that was added.
Gerald H. Jacobs, Gary A. Williams, Hugh Cahill, and Jeremy Nathans. Emergence of Novel Color Vision in Mice Engineered to Express a Human Cone Photopigment. Science 315 (5819), 1723. (23 March 2007)