Extension for material properties?

[N. W. Perry]

The only appropriate thing I can think of would be slope bumps

There are a couple of other plastic finishes, most notably the less glossy finish found on the softer plastics used for pointed parts. Maybe some other "frosted" surfaces, and I believe there are a few different grades of roughness for the slopes as well. These might well all fall under the same set of parameters as the slope surface.

Otherwise, printed/sticker/ink surfaces also tend to have a duller finish. But these also have their own peculiar colors, so maybe those are kept within the !COLOUR meta.

Keep it simple. Something like:
0 !SURFACE SLOPE|(Other surface finishes here)

I guess the specific parameters would be defined by the individual materials—in other words, just the same way as the MATERIAL tag is now.

How those surfaces are rendered would be up to the author of the program that implements the META.

By coding I meant the numerical code. In other words, if the code for the slope material is 00100, then a yellow slope would get color code 1400100. But maybe it should be 14-100, or 14.100, or not use numerals at all…