The short explanation
Standard HDMI video is 8 bits per channel: 24 bits per pixel, about 16.7 million colors. Deep color extends that to 10, 12 or 16 bits per channel — up to 48 bits per pixel.
Where you run into it
The gain is smooth gradients. At 8-bit, a sky or a slow fade steps visibly; at 10-bit it does not. HDR makes this critical, because a wider brightness range spread over 256 steps bands badly.
It costs bandwidth in direct proportion, which is why deep color and high frame rates compete for the same budget. It also has to be advertised: a chain that does not declare deep color gets 8-bit, and HDR either fails or bands.
How the DR HDMI 8K handles it
The DR HDMI 8K supports 4K at 10, 12 and 16-bit and up to 48-bit deep color, and lists deep color support explicitly in its specifications. Its EDID tables let you advertise the bit depth deliberately — 10-bit for 4K120, 12-bit for 4K60 4:2:0 — and the OLED shows the color depth each EDID declares.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.