Why External monitor's input color format shown YPbPr (isn't RGB)?

FPlayApple wrote:

Why External monitor's input color format shown YPbPr (isn't RGB)?

When I connected Mac mini m1 to external monitor (Dell S2721QS) via HDMI.

if in doubt search the developers website or contact their: Support/Help/FAQ/Known issues/compatibility/Updates

Support for Dell S2721QS | Drivers & Downloads

Support | Dell US

I have seen this reported elsewhere in the past with Dell monitors. It appears to be the monitor's default color space.

FIRST, using the monitor's On Screen Controls, set the Input Color Format to RGB. That may solve the problem.

If that doesn't work, my suggestion is to connect via Displayport instead of HDMI, but you will need an adapter to do this. (HDMI to Displayport or USB-C to Displayport). This will force RGB mode.

FWIW, I have never been enamored of the Mac Mini's video rendering over HDMI as far back as the 2014 models. It was always off-color and fuzzy regardless of the display or TV that I used. The solution was to use DVI or DisplayPort instead of HDMI. The video image is outstanding that way.