Hi Ron,
I asked this same question previously and it is average (not additive).
I have both the mono and the colour Horizon cameras. With the mono I use 2x2 binning all the time but with colour versions it depends on what the use case is? Apologies if the following is already something you know -
For example, if you just want live images and save colour pictures 'as seen on the screen' then 2x2 is great and it works in colour.
However, if you want to save data in RAW fits format for subsequent post processing then ''don't use binning'' (which means the same as ''1x1 binning''). This is because the data is saved as mono (not colour) which then has to be converted to colour later (debayered). However, 2x2 binning destroys the ''colour filter array'' pattern on the cmos chip so you can no longer retrieve the colour image.
I made this mistake but now I avoid that problem.
Apologies if you know all this!
David