What can be done about unsightly marks left on my legs from a liquid nitrogen treatment?

Our Hidef protocols for scar revision can address different types of scars, depending on your scar biometrics and characteristics. Protocols vary in duration based on the severity of your scars. 
Bad scars can manifest in many different ways, including pigmentation, thickening, and depression of atrophic scars. Thick scars are best treated in my practice with IIT, cortisone injections, pressure management, and laser. We recommend daily use of Plato’s Scar Serum to the scars to mediate scar hypertrophy. 
Hyperpigmented scars respond well to Melarase AM and Melarase PM creams. If the area is large, consider Luminase and Exfolase creams for the body. Hyperpigmentation PIH can also be treated with Q switch laser, PICO, chemical peels, and rapid scar exfoliation. PIE erythema scars are best treated with pulsed dye laser. 
Keloid scars are our specialization. We manage small and large keloid scars with excision, IIT, PRP, laser therapy, and scar modulation. Use Plato’s Scar Serum daily. 
Textural scars respond well to nanofractional RF and fractional laser to help improve surface texture. When combined with scar modulation, the results can improve dramatically. The most important variable is for you to choose a plastic surgeon with expertise in laser therapy using Hidef protocols. 
Best,  
Dr. Karamanoukian  
Realself100 Surgeon

These spots can be improved with a combination of Cosmelan, lasers, Aerolase, microneedling/prp, skin lightening, peels, and skin care. Sun protection is essential. I recommend getting a formal evaluation with a scarring expert. Best, Dr. Emer

Any treatment to the skin can cause the skin to hyper pigment.  The darker your skin tone the more likely this will happen.  Usually this pigmentation will fade over time.  There are prescriptive products, laser and light based treatments that can help speed up the process.  You should contact your physician and let them know about your concerns.