An old post salvaged from the dying Google+
I used to think diagrams like the above were probably accurate in showing dichromats, like dogs, having a gradient between only two qualitatively distinct colors at most (of course, what those particular colors are qualitatively is unknowable; surely they don’t feel like our blue and yellow). But from Wachtler, Dohrmann, & Hertel (2004) I learned that human dichromats claim to see not a smooth continuum, but multiple qualitatively distinct bands.
Some of the qualitatively different colors we see in the spectrum reflect post-receptoral mechanisms that dogs might also have. If so, you should be able to find a boost in dog learning performance (but not discrimination, which reflects the receptor space) for two colors on either side of a band boundary. Don’t know if this has been done.
Comment by Jonathon Cohen of UCSD: A really interesting paper arguing for a particular version of this kind of revisionism about the standard line on what the world looks like to dichromats is Justin Broackes, “What Do the Colour-blind See?”, in J. Cohen & M. Matthen, eds., Color Ontology and Color Science (MIT Press, 2010). Highly recommendable.