Erythrasma is common in the armpits, under the breasts, in groin folds, and between the toes. Depending on the skin tone, the rash is reddish or darker colored and looks coral/red under a black light.

Armpit pain is a common complaint that can ... A contact dermatitis rash is often red or purple and can be itchy, burning or painful, and develop blisters. Irritant contact dermatitis results ...

Dark underarms may simply have to do with pigmentation and levels of melanin in the skin. But sometimes armpit discoloration ...