We would like to serve data as they are, without uncontrolled changes, because it is almost impossible to set color balance to one fitting all places in the world and all groups of users. You can still tweak contrast in several ways.
You can tweak “Gain” (brightness) to automatically equalize the image.
You can tweak “Gamma” (contrast) as well.
Use the atmospheric correction by using the S2L2A atmospherically corrected data instead of S2L1C. As you can see on the image below, atmospheric correction increases the contrast, as it corrects for the effects of the atmosphere.
