color setting for bpp = 8 ?

As far as I can see, possible bpp values are 1,2,4,8,16,24,32
In set Color bpp of 16,24,32 are handled seperately
All others result in 0 or 0xFFFFFFFF only
Is there a technical reason for that ?

Are you sure about that? Maybe I'm misunderstanding the question.

In 16/24/32 you get R,G and B channels handled separately (if you specify them), but in anything else it's considered that the image is a single channel, so setColor(123) for a BPP of 4 would write the value 123.

Hmmm, may be I'm misunderstanding the answer.
First, setColor(123) sets color correctly to 123.
Why is setColor(0.5.0.5.0.5) not supported for bpp = 8 ?
If this is on purpose a short hint in description would help.

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...