I finished looking at all the applets and reading all the text. I think the applets are marvelous, and the text quite good.
Of course, I have a few quibbles.
the reference white is set for you by the manufacturer, typically to a slightly blueish-white called D6500.
They mean D65, not D6500. Actually, D65 defines a spectral power distribution, not a color, but you can turn it into a color by specifying an observer.
In closing, one question you might be wondering about is: how were the X, Y, and Z axes chosen? They were clearly chosen to ensure that the spectral locus fell into the all-positive octant of XYZ colorspace. Beyond this, the answer is complicated and beyond the scope of this applet; the interested reader is referred to (Wyszecki 1982)
I consider referring non-color scientists to Wyszecki and Stiles to be cruel and heartless. There are many more approachable books, although W&S is an indispensable reference if you really know what you're doing.
It’s great that they stayed in three dimensions as long as they did. I think they should have talked about the math to get from rho,gamma,beta to XYZ (not at all difficult for any Stanford student), and they should have defined xy and u’v’ chromaticity space instead of talking about "XYZ chromaticity space".
I’m disappointed that the gamut mapping exercise was in two dimensions, especially since printers are explicitly called out. They do note and sort of apologize for this, however.
I love the way they make it easy to create 4 and 5 primary simulated emissive displays.
None of these comments are meant to take away from the worth of the material. It is very well done.
Jim