Pimple marks can develop in different ways:
Reddish marks (PIE - post-inflammatory erythema) occur when the fine blood vessels remain dilated after an blemish - usually on the face, décolleté or skin on the back.
They are particularly common if pimples have been squeezed or the skin has been more severely damaged. UV radiation can also intensify skin discoloration and make it more difficult to remove. Unlike acne scars, pimple marks are not indentations or unevenness, but flat discolorations of the skin surface.
Dark pimple marks (PIH - post-inflammatory hyperpigmentation), on the other hand, are caused by increased melanin production as a result of irritation - often on the face, décolleté or back.