What is state of the art in multivariate 2D image display?
If "kind of works" but slightly less ugly is acceptable, than any two not-too-near-to-co-linear RGB color space vectors will do, and you or the user can just play with them to get a preferred combination of differentiation and non-ugliness (say, from a suitable artistic palate coordinated with other artwork color schemes in or around the data presentation, etc.) for the particular data sets.  Perhaps let the user choose one color vector, and auto rotate the other color vector to some roughly orthogonal point in the color space if it's too close to co-linear.

If you invert the data vector (or use a subtractive color vector) and thus fade to white, the lighter pastel version of some color vectors may blend better, but this depends on the mean and distribution of the data.