Hello,
First of all you should wait for pityriasis rosea to run its course and then if there are any dark scars then they have to be treated on lines of post inflammatory
hyperpigmentation. The treatment for post inflammatory hyperpigmentation includes various skin creams like hydroquinone, azelaic acid, corticosteroids, tretinoin cream and laser treatment. If it still persists then you can go in for laser treatment, Chemical peeling or microdermabrasion.
I hope it helps.