Sorry but that’s not the case.
RGB and YUV420 here are “color modes”, or to be more precise, “compression modes”. RGB provides fully uncompressed images, while YUV420 provides 1/2 compressed images.
The YUV420 mode is there because some old televisions, though support HDMI 2.0 metadata, do not support HDMI 2.0 bandwidth of 18Gbps. 2160p RGB requires at least 17.82Gbps and therefore cannot be used on those old televisions. However, those televisions are highly likely to support HDMI 1.4 bandwidth of 10.2Gbps, so that 2160p YUV420 (8.91Gbps) may work on them. Again, this has nothing to do with HDR.
Choose always Auto or 2160p RGB if your TV supports. It always gives you better images than 2160p YUV420.