color setting for bpp = 8 ?

Ahh, ok - yes, It's not supported because there's no standard way of mapping R,G and B to a single 8 bit value (same with 1, 2 and 4 bits). About the only thing you could reliably assume is white being all 1 and black being all 0 (or vice versa) which is what it does.

I've updated the docs here: https://github.com/espruino/Espruino/com­mit/8ef686d5f405abd5ed76e80dbaa8930c49b9­4f69

If you have some specific display that works another way, nothing stops you from just overriding setColor with your own implementation though...