fillcolor

     splot FOO with pm3d fillcolor <colorspec>

Plot style with pm3d accepts an optional fillcolor in the splot command. This specification is applied to the entire pm3d surface. See colorspec. Most fillcolor specifications will result in a single solid color, which is hard to interpret visually unless there is also a lighting model present to distinguish surface components based on orientation. See pm3d lighting.

There are a few special cases. with pm3d fillcolor palette would produce the same result as the default pm3d palette-based coloring, and is therefore not a useful option. with pm3d fillcolor linestyle N is more interesting. This variant assigns distinct colors to the top and bottom of the pm3d surface, similar to the color scheme used by gnuplot's hidden3d mode. Linestyle N is used for the top surface; linestyle N+1 for the bottom surface. Note that "top" and "bottom" depend on the scan order, so that the colors are inverted for pm3d scansbackward as compared to pm3d scansforward. This coloring option works best with pm3d depthorder, however, which unfortunately does not allow you to control the scan order so you may have to instead swap the colors defined for linestyles N and N+1.