Ms. Whitelaw's wonderful work has a great clarity to it, a clean feel like that often rendered in watercolor -- something of a dreamy quality, where your imagination or recollections are stimulated by visual clues (like the power of scents).
Some might say that such "purity" works particularly well for "innocent" subjects, like nature or children. But it can also for the portraits of powerful adults, as in this limited palette...
http://www.nga.gov/cgi-bin/pinfo?Object=46245+0+none
I'll bet Ms. Whitelaw's #6 is a black (as in the boy's hair or the tree trunk in the landscape on her homepage).
I'd worry, of course, about the permanence of a napthol red (but not a cadmium).
Limited palettes usually work well on one side of each primary -- for her choice of colors, one would expect the cool side of yellow; the cool side of red; and the warm side of blue -- although you don't get the feeling of any "omissions" from her work. I think that's the play of the mental "cues" she gives you, as I mentioned -- your imagination or recollections fill in the rest (as I believe Karin put it, your mind seeks out balance in the colors of a composition) -- color is, as we've said, a psychological as well as a physical phenomenon.
Here are some other rather interesting palettes (take a look at the bottom of the page)...
http://gamblincolors.com/howtp.html
I've "fallen in love" with their transparent earth shades -- more like the Old Masters' earth pigments than modern siennas etc.
Incidentally, the original question I had was about the limited palette used by printers and photographers (who ultimately "hold all our works hostage") -- cyan, magenta, and yellow -- that's blue on the cool side, red on the cool side, and yellow on (I believe) the cool side. I don't know what all that means, but it's interesting all the same.