I needed to solve this problem about 12 years ago: Digging through the scraps of Fortran available on then-much-younger Internet (I may have even resorted to t

I needed to solve this problem about 12 years ago: Digging through the scraps of Fortran available on then-much-younger Internet (I may have even resorted to the university library!), I discovered that a standard trick for coloring 1-dimensional data onto a (Red, Green, Blue) color space is to start in ([H]ue, [S]aturation, [V]alue) space, map your data into the H dimension (holding S and V constant), and then transform that into RGB. I have absolutely no expertise in this field at all, but that Worked For Me (and does indeed place purple between blue and red).