For your setup you could just create a "color_BC.png" for each of your 8 colors as a 128x128 image with paint or a similar program (just solid color) and add them to the zip. Your cubes might also need (arbitrary) UV maps, don't know how the game behaves if there are no UVs at all. The colors should then show ingame. If that works you can start experimenting with roughness maps etc.