ElumTools derives the reflectance of each element based on its Material’s Graphics Shading Color property. This can be easily overridden in ElumTools Material Mapping dialog, without otherwise changing the existing Revit model.

ElumTools calculates the surface reflectance from the Red, Green, and Blue (RGB) components of the material "Graphics Color" in the Revit materials section. The relationship between grayscale reflectance (overall reflectance) of a surface and its RGB color equivalent is as follows:

Grayscale Reflectance = 0.2125 * R + 0.7154 * G + 0.0721 * B

where R,G and B are expressed as numbers between 0 and 1.0 (when RGB is expressed by numbers between 0 and 255, divide by 255).

For additional information, please see the help article, and watch the video.