DIAGNOSIS That's a stuck pixel, not a dead pixel. It's receiving a signal but locked on one color, often due to a transistor briefly sticking.
FIX Use a soft, microfiber cloth. Apply gentle pressure with your fingertip (through the cloth) directly on the stuck pixel for 15-20 seconds. Power the monitor off while doing this. Power it back on. This can sometimes re-align the liquid crystals.
IF THAT DIDN'T WORK Run a pixel-exercising video (search 'LCD pixel fixer' on YouTube) full-screen for 1-2 hours. The rapidly cycling colors can unstick it. Avoid software 'massagers' that require installation.
HEADS UP Dell's pixel policy typically requires multiple dead/stuck pixels (often 6 or more) for a warranty replacement. A single pixel, while annoying, is rarely covered.