For stepup or for the scripts? For the scripts there is a shaderColors.py file in the _tools directory.
For stepup it is hardcoded in kicadStepUptools.py starting at around line 1500 (plus or minus a bit depending on version. But you could simply search for the line as_is="")
Defining the color values is a bit harder but there is a tutorial out there.
Edit: found the tutorial: Tutorial and guidelines for better materials on 3D models