The current default colormap that yt uses is called "Algae," and it's been with us for a while. Recently some of us on the yt-dev mailing list (which is open to join!) have begun to look at alternate colormaps that have better characteristics, particularly from an accessibility perspective.
If you have the time, it would be really, *really* helpful if you could take this anonymous, two question (only one of which is mandatory!) poll about which of the four candidates we've come up with you like the best.
Thanks very much! The poll will be open at least a week, and I'll report back the results here sometime after that.