DIAGNOSIS The monitor's ComfortView or Color Preset mode is active. These are factory-set features intended to reduce blue light.
FIX Press the monitor's menu button. Go to Color > Preset Modes. Select 'Standard' or 'Custom Color'. Then, go to Color > ComfortView and set it to 'Off'. This will revert to the default, cooler color temperature.
IF THAT DIDN'T WORK Perform a factory reset. Go to Menu > Others > Factory Reset. This will wipe any inadvertent color calibration changes. Also, ensure the 'Input Color Format' under Display settings is set to 'RGB', not 'YCbCr'.
HEADS UP The U2723QE uses an IPS Black panel which has higher native contrast and can appear richer/warmer than standard IPS panels. Give your eyes a day to adjust before deciding it's wrong.