Diagnosis
The monitor is likely running in a default 'Standard' or 'ComfortView' preset with low contrast and a cool color temperature. The default factory settings are not optimized for vibrancy.
How to Fix It
Open the OSD with the joystick. Go to Display > Preset Modes. Select 'Game' or 'Movie' as a starting point. Then, manually adjust. Go to Color > Custom Color. Set Contrast to 75%. Set Gamma to Mode 2. Set Color Temperature to Custom RGB and set values to R=97, G=99, B=96. This provides a more balanced, richer default.
If That Didn't Work
The source is a factor. On your PC, open display settings (e.g., NVIDIA Control Panel or AMD Radeon Software). Ensure output dynamic range is set to 'Full' (0-255), not 'Limited' (16-235), which crushes black detail.
Heads Up
The S2421HS uses an IPS panel. IPS panels have superior viewing angles but inherently lower contrast ratios than VA panels. You will not get inky, OLED-like blacks. This is a panel technology characteristic, not a defect.