Chrysalis: how do you unset a key colour?

I can set a key to a blank palette slot, but it’s not clear how you unset a colour on a key.

There is no such thing as unsetting a key colour, really, not technically. You can set it to a black palette slot, and that’ll turn the LED off - that’s what is being done by default, too: out of the box, all keys on the colormap are mapped to palette index 15, and all palette colors are black.

Probably unlikely to happen, but if you used all the palette positions would that mean you couldn’t reset a key?

Indeed, if you used all positions and none of them were black, then you couldn’t turn LEDs off, not while Colormap is active.