Davidjones1:
Surfaces dialog box, Engine Settings: Open GL, Exposure to Light panel > click on Surface Color. On the Mac the color picker will appear, select the Sliders icon at the top and RGB Sliders in the pop-up list.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David