In the current "set palette" implementation, the lower or upper values out of cb range are colored
with the lowest or uppermost color in the given palette for the image plot (or also pm3d plot).
The requested features are the new keywords "lowercolor COLORSPEC" and
"uppercolor COLORSPEC" for "set palette" to specify the colors for the lower and upper values
out of cb range.
I think that the different colors for the values out of cb range could be used to distingwish
the masked pixel with the valid pixels in image plot (or also pm3d plot). It also can be used
to indicate the saturated pixels in image plot.
The following is a sample code I think,
set palette defined (0 "white", 1 "red") lowercolor "gray" uppercolor "dark-red"
set cbrange [0:1]
plot "image.dat" with image
I think these keywords could be replaced by other appropriate ones.