Post-Inflammatory Hyperpigmentation, also known as PIH, is one of three major types of hyperpigmentation. The other two, sun damage and melasma, can be read about in this FutureDerm article here.
Post-inflammatory hyperpigmentation looks like brown or darkened spots, but rather than being caused by the sun or hormonal changes, it is a result of an injury to the skin, and usually from various conditions (acne, eczema, laser treatments, too-strong chemical peels, etc.). It can affect people of all skin tones, but has a greater propensity towards darker skinned individuals. For instance, a study in Lasers in Surgery and Medicine revealed that PIH resulted more commonly in Asians than Caucasians after laser treatments.
The key to treating PIH is to first remove the causative agent. This means cessation of all inflammatory or irritating factors, including eliminating excessive light and heat exposure, super-strong skin care agents or treatments, and as many stressors as possible. This also means piling on the sun protection and soothing hydrators, like oatmeal extract, aloe, and calendula oil.
The second step to treating PIH is to select anti-hyperpigmentation treatments that are not irritating to the skin. While retinoids, glycolic acid and other AHAs, and hydroquinone are all considered first-line treatments for hyperpigmentation caused by UV damage or melasma, you need to take a gentler approach when treating PIH. I recommend (in decreasing order) licorice extract, brightening peptides, arbutin, mulberry extract, bearberry extract, and soy. Even vitamin C, which I consider to be a gold standard of skin care when used in high concentrations, may not be as highly-recommended when used to treat PIH. (I personally keep it to 10% or less when PIH is involved.)
One product I think is solid for PIH (and no, this is not a paid endorsement) is IT Cosmetics Secret Sauce™ Moisturizer. It contains proven ingredients in significant concentrations: brightening peptides, vitamin C (in moderate concentration) and licorice root. Using it for at least 4-6 weeks nightly, allowing for cellular turnover to occur, should start to lighten the appearance of dark spots/hyperpigmentation.
Another product set I think is quality for PIH (also not a paid endorsement) is the TATCHA Luminosity Set. It is quite gentle to the skin, with as many healing factors like green tea and oatmeal as brightening agents like vitamin C and licorice extract. It might actually take a little longer to work than, say, the IT Cosmetics product, but the set will also soothe and soften your skin. (The packaging is beautiful, for a little inner boost, too)!
Bottom line — post-inflammatory hyperpigmentation is not the same as regular hyperpigmentation caused by UV light or melasma. In the case of PIH, make sure you discontinue all potentially causative agents and treatments, pile on the UV protection, and start using brightening agents in mild to moderate doses/concentrations.