Blue and/or red light therapy works by reducing acne bacteria and skin oil, and can be administered in weaker strength using at-home devices, or in higher strength using in-office devices in a doctor's office.
At-home devices: Evidence of effectiveness is weak when using at-home devices, but they may work to some degree. The biggest upside is that they have few side effects. The biggest downside is that you have to shine the light on the skin every day for 30-60 minutes, which can lead to people leaving light devices unused in the drawer or closet. Warning: Red light can harm the retina. Always wear goggles when using red light or red + blue light devices.
In-office devices: Evidence of effectiveness is more compelling when using stronger light therapy devices in doctor's offices. These devices likely reduce skin oil production and can reduce acne to a measurable degree, albeit not completely clearing acne. The biggest upside is that results may stick around for months after a course of several treatments. However, a concerning downside is that if they do indeed permanently reduce skin oil production, this might led to unintended consequences like premature aging of the skin.