Color Mixing with channels additional to RGB

All colors are used when mixing using the Color Picker or Color Chips.

If we have color measurement data for the fixture, the console use that to create a reasonable match to the color on the picker/chip.
(As long as the fixture is physically capable of producing the color.)
It's not as close a match as Eos is capable of doing, as this is a smaller console with rather less CPU and memory.

If we don't have that color measurement data, then the console makes an estimate of which specific LED colors the fixture is using.
This will be a much less accurate match of course. In recent updates (2.0, 2.5) we've been improving the algorithm used to make these estimates.

The console makes a 'rich' mix - as bright as possible and using as many emitters as possible.

The Direct color mixing controls are so you can remove colors from the mix and make your own (less bright) metamers for specific purposes.

Presumably only Direct colour mixing can bring the UV emitters into play

Yes, that's correct. UV is always assumed to be a 'deep' UV (shorter than 380nm) and thus not a visible color.
- Not in the CIE 1931, 1960 and 1976 chromaticity diagrams, for those of you reading the ETC et cetera blog

While most LED "UV" is longer than that, and so visible, it's usually so dim relative to the other colors that it'd be meaningless to bring it up to make a better indigo or magenta.