[noggin]

 I do not think the shield supports resolution switching.
And I do not see the big differences between having the upscaling to the shield rather than the TV.
So I prefer to use a fixed resolution 2160p (59,940 o 60Hz) for bot 4K and NO 4K videos.

but I do not know how to set the color space.
for 4K i thinks YCbCr 4:2:0 10Bit rec. 2020 is OK (recommended on nvidia shield stttings tab).
For NON 4K videos? YCbCr 4:2:0 10Bit rec. 709??   

When watching Rec 2020 UHD content - you want a Rec 2020 gamut output
When watching Rec 709 HD content (and Rec 601 SD upscaled to Rec 709) you want a Rec 709 gamut output, as neither Kodi nor Android currently convert 709 gamut content to 2020 gamut.

The Shield TV doesn't automatically switch between 709 and 2020 flagged output - so you need to manually switch in nVidia Sheild TV Android settings, which is a bit dull after a while...  

Thanks.
And about bit and YCbCr mode? 

That will depend on your content sources and your TV and AVR (if you have one) properties.  Not all source material can be played in the same format.  

RGB and YCbCr 4:4:4 are only available for <30p frame rates at 2160p for HDR content (which requires 10-bit output as a minimum). Almost all content that is UHD HDR is <30p - but there is some 50p and 59.94p stuff knocking around now.
For >30p stuff you are limited to YCbCr 4:2:0 or YCbCr 4:2:2 - and the latter is only supported at 12-bit output for >30p output.

All consumer content is 4:2:0 at source (SD, HD or UHD) but I don't know if you output 4:2:0 whether Kodi has done a conversion to 4:4:4/RGB and then a second conversion...