DIAGNOSIS The monitor is likely stuck in a limited color space mode, like sRGB, or the Black Equalizer setting is set too high, crushing detail.
FIX Access the monitor's OSD. Navigate to Picture > Color Space and set it to 'Custom'. Ensure Gamma is on Mode 2. Set Black Equalizer to 13 (the default). Go to System > Input Signal Plus and ensure it is 'On' for the port you are using (HDMI or DisplayPort). This enables the full bandwidth signal.
IF THAT DIDN'T WORK The issue may be source-side. On Windows, open Display Settings > Advanced Display > Display adapter properties for Display 1. On the Color Management tab, ensure 'Use my settings for this device' is unchecked to let the monitor handle color.
HEADS UP The monitor's default 'Dynamic Brightness' setting in the Eco Solution menu can cause erratic brightness and contrast shifts. Disable it completely for a stable image.