[noggin]

I am not too much interested in workarounds as the Firetv 4K worked flawlessly without any issues at all. They should fix their stuff. So now I have speedy menus but that's it, speedy menus with wrong colors.

Guess what outcry would happen if we would fuck up the colors that much here in kodi ...

Agreed.  Recently picked up a 2019 Cube to try and I have been very disappointed.  The device is quick, like really quick, but the BT.2020 colour space issues when attached to an AVR are unacceptable.  The device never switches to Rec. 709 when set to Adaptive Dynamic Range.  8-bit, Up to 10-Bit, Up to 12-bit, Auto Colour Space, RGB, YCbCr - all of them behave the same - SDR is output using the BT.2020 colour space.  When connected directly to the TV the issue isn't nearly as bad but its still there.  When connected to an AVR the colours are all sorts of wrong.  The Fire TV Stick 4K is a night and day difference with colour space switching that works and behaves properly.

Kodi (using latest fritschfiretv build) also doesn't seem to function correctly.  Skipped ahead or back causes most files to freeze up while the playback timer continues and audio seems to take a second or two to sync up and sometimes doesn't work at all requiring a stop/start of the file.  Haven't spent too much time trying to troubleshoot it because with the colour issues its not really worth the effort.

Raised a service ticket with Amazon Customer Service and after an hour or so of going back and forth they decided to escalate the colour space issue to the engineering team.  Said it would be a week or two before they heard back but they would update me via email once a response was received.  Hopefully they can reproduce the colour space issues, should be pretty easy, and fix them so that this becomes a serviceable Kodi device.  As is I wouldn't recommend it over the Fire TV Stick 4K.

This isn't the issue we used to see with nVidia Shield TVs is it - prior to them offering a Rec 709 / Rec 2020 switch - when they used to output all Rec 709 or all Rec 2020?  

They used to correctly map Rec 709 into Rec 2020 colour space in SDR and output with a BT.1886 SDR EOTF flagged - which caused a lot of TVs to display somewhat differently to the same material output Rec 709 without an EOTF flagged, which the TVs then displayed with a Power Law Gamma (2.2-2.4) rather than a BT.1886 SDR EOTF (which is only power law for some of the range)

Or is this Rec 709 video output as Rec 709 - but flagged as Rec 2020 without any mapping? (i.e. the colours are so off as to be ridiculous with luminous greens and retina burning reds and oranges etc.)

(Rec 709 can be mapped perfectly well into Rec 2020 colour gamut - after all your TV has to do that to Rec 709 content to display it on a Rec 2020 display - however Rec 2020 has a properly specified SDR EOTF, whereas Rec 709 - and Rec 601 before it - didn't really.  This is very different to the reverse case - where you have to tone map wider gamut Rec 2020 content into the reduced gamut of Rec 709)