How to get rid of discoloration from a scar?

Depends. On how much money you are willing to spend. Once the scar tissue is there, little options exist outside cosmetic laser therapy with dermatology, likely cosmetic and expensive. Best is to prevent scarring. Keep tissue away from sunlight and use sunblock 50 SPF or higher when outside esp on face. Mederma can help prevent scarring. Hydroxyquinone creams can help lighten some areas.
OTC creams or Doctor. If new and fairly small, ask your pharacist to recommend an OTC cream. They work well. But if not new or large, consult w/ a dermatologist - even a scar specialist to discuss possibilities. Best wishes!