What does the half moon at the base of the nails mean?

As for color variations—bluish, gray, purplish, or even reddish—they can indicate poor blood oxygenation, a reaction to medication, or simply… a cold. Artificial lighting or a slightly numb hand can also deceive the eye. However, if this strange hue persists, medical advice is required.
A stained or very red lunula, in particular, warrants an appointment with a dermatologist, if only to rule out infection or a vascular disorder. The important thing to remember is that the lunula evolves slowly. Changes are rarely observed overnight—it’s therefore best to monitor its appearance over a few weeks, in natural light, and in complete peace.