Tinea versicolor. This is most consistent with tinea versicolor (not tinea corporis or "ringworm") which is caused by a yeast species called malassezia. It starts in the scalp (where it is invisible), then spreads to neck, chest, & back. Treatment is an antifungal shampoo (nizoral, loprox, & others) + an antifungal cream (loprox, naftin, (naftifine) many others). Selsun blue does not work, despite what many web sites say.
Tinea corporis. The is a skin infection that can cause both light and dark patches on the skin. Non- resaised and not the usual raised lesons with central clearing that people know as "ringworm". These spot are usually of the chest and back, commonly the same areas that are stained with sweat when working out. This is fungus that can be treated with non perscriptions anti fungal creams and head and shoulders.