Hi, I can understand your concern for purple-red spot on forearm. There can be multiple possibilities of the purple-red spot and it requires a close physical examination to diagnose the exact entity of the problem. In my view correlating with the facts you have provided it appears to be simple post inflammatory pigmentation. If I were your dermatologist then I would have suggested you to use azelaic acid cream or Kojic acid cream or glycolic acid cream or licorice extract or arbutine creams at night time in guidance of dermatologist. The most important part is to use a sunscreen regularly in open environment and on exposure to sun. Take care All the best