I have found while using the "pm3d" surfact colouring
options that I frequently would like to colour the
facets based on the "highest" or "lowest" corner value.
Choosing one of c1|c2|c3|c4 gives potentially odd
results; if the data forms "serrations", the two sides
of the serrations are coloured differently, as c1 (or
whatever) are at different heights.
I would therefore propose adding "top" and "bottom" to
allow colouring based on the highest/lowest point (in z).