Red and purple stretch marks, explained
Red stretch marks are new stretch marks. The colour comes from blood vessels in the healing dermis, and the NHS says marks usually start off darker and fade over time. Clinically they are called striae rubrae. Stretch Mark Removal London explains why this is the stage when treatment works best, and also the stage when waiting costs nothing.
Over roughly six to twenty-four months a red or purple mark flattens, loses its colour and settles into a paler line. That fading happens without any treatment. Treatment during the red stage tends to change more than it will later, because there is still an active repair process to work with, but it also takes credit for fading that was coming anyway. The guide to types of stretch marks compares red and white marks.