Background: As you know, the first command gives a nice spectrum; the second mixes red and blue paint so we see a muddy purple at midrange:
1) set image colorscale hsb 0 1 1 .max. hsb 0.6667 1 1 .min.
2) set image colorscale rgb 1 0 0 .max. rgb 0 0 1 .min.
QUESTION: Is there an easy way to traverse from red to white to blue?
(That color ramp has merit (see colorbrewer2.org). By "easy" I mean without laboriously specifying many RGB values.)
Just because it is laborious doesn't mean it hasn't already been done!
I have posted a red-to-white-to-blue colorscale at http://homes.comet.ucar.edu/~codyk/red_white_blue.color. You would use it as you would any other colorscale read from a file, e.g.:
read image colorscale
I can't guarantee that the file will be available at that link forever, but it will certainly be there for a few months (in case anyone stumbles upon this thread in the future).