So far, only min and max colors are supported (lesson 15).
...
r.onShowtime = function() {
v.minColor = [0, 0.06666666666666667, 1];
v.maxColor = [0.5843137254901961, 1, 0];
};
...
You could also use a custom labelmap/color table as a hack for now (lesson 11).
There is no easy mechanism to support it right now but this is on the TODO list and should not be too hard to implement.
Please feel free to contribute to XTK on github, this would be a very useful addition! https://github.com/xtk/X
Thanks