DIAGNOSIS The monitor's HDMI port is defaulting to the wrong color format or limited dynamic range for the console signal.
FIX Change the HDMI version setting on the monitor. Press the joystick. Go to Menu > Display > HDMI Version. Set it to 2.0. Then, go to Menu > Color > Preset Modes. Set it to Standard or Multimedia.
IF THAT DIDN'T WORK Check the console's output settings. On the Switch, go to System Settings > TV Output. Set RGB Range to Full Range. Also, ensure Resolution is set to Automatic, not 1080p forced.
HEADS UP The monitor's Game and Cool/Warm color presets can distort colors for consoles. Stick with Standard or Multimedia preset for accurate color until you manually calibrate.