I don't know *why* I'm seeing what I'm seeing, all I can say is that over the weekend I made the following two changes to my setup:
1) I updated my OPPO 203 to this latest-and-greatest firmware
2) Due to some instabilities/glitchy behaviour coming out of that firmware update, I did the "settings reset" (without also resetting my account information) that was recommended multiple times in this thread.
I have my nvidia shield connected to my OPPO 203 via HDMI in. Prior to doing those two things, if I had my Shield set to output 4k 10-bit 4:2:0 BT.2020 and then I streamed some 1080p SDR content like Dinotrux on Netflix (a children's cartoon with very vibrant animated colors; tye's reds are very bright red, etc), I would experience the "muted colors" that many often complain about when the Shield is set to exactly this sort of output and then used to stream SDR content. Tye's "reds" would be very muted, browner, almost taking on an orange tint. If I simultaneously streamed it on my Samsung Galaxy TabS tablet and held it up to my Sony 940E for a side-by-side comparison, the differences in the colors was stark. If I switched my Shield over to 1080p 8-bit 4:4:4 BT.709 (again, prior to this weekend), Tye's reds would look (to my eye) exactly the same as what I see on my tablet.
Since I did those two things this weekend, the colors I see when I stream Dinotrux with my Shield set to 4k 10-bit 4:2:0 BT.2020 look an awful lot like what I see on my tablet, which is an awful lot like what I see when my Shield is set to 1080p 8-bit 4:4:4 BT.709... the "issue" I was having (which many others who don't own OPPO 203's have shared) of washed out colors when viewing SDR content while set to an HDR output has basically disappeared.
I don't know why.
a) I don't know if it's something that was included in this firmware update.
b) I don't know if I was just multiple firmware versions behind prior to this weekend, and such a tweak/improvement was included in a previous update that I just never took.
c) I don't know if the act of resetting my OPPO/settings corrected something nefarious. Perhaps a user error on my part with my previous settings/configurations. That's possible. I personally think it's unlikely, but I can't rule it out.
I know @groove93
had been saying for a while now that he didn't see these color space problems with his Shield (which is also connected to an OPPO 203, which is connected to a very similar Sony tv as mine), and I'd never figured out why, but if the answer to my "why" question isn't a) but rather b) or c), I see now what he's been saying all along...
Tonight I'm gonna try to connect my Shield directly to my TV (bypassing the OPPO) and try to prove to myself that it is the OPPO that is in fact "adding value" here. Whether or not the value was added *this weekend* via this specific firmware update or not doesn't really matter to me... would be a FANTASTIC extra perk to owning a 203, though, if I could prove to myself that it's correcting another device's color space output problems the way I think it might be...