Here is the example in http://gnuplot.sourceforge.net/demo/pm3dcolors.html
set palette rgb 21,22,23; set title "hot (black-red-yellow-white)";
The above code set the hot palette. However I want a reversed hot palette, say:
white-yellow-red-black.
Small least value map to white and largest value map to black.
Use negative numbers to invert the palette:
set pm3d map
set palette rgb 21,22,23
splot x
gives you
whereas
set pm3d map
set palette rgb -21,-22,-23
splot x
gives you