I have scraped my leg under my knee last year and it has healed but the color of is darker than my skin color is that a scar? is use of mederma good ?

Improve not remove: Yes this is most likely a scar. It sounds like post inflammatory hyperpigmentation that will most likely improve with time. However other beneficial treatments with various interventions such as silicone gels, sunscreen, avoidance of tobacco use, peel, bleaching agents, microdermabrasion, dermabrasion, laser or ipl treatments, and many more options.

Dyspigmentation: It is something called post inflammatory hyper pigmentation. This can occur after an injury. It may fade over time. Sun protection is important. Sometimes the discoloration can be permanent. There are bleaching creams we could try but i would see a dermatologist.