Sentinel-2 images color cast

Blue color cast is caused by atmospheric scattering (= light reflecting off particles in the atmosphere is added to the light reflected from the ground), and it is always present in top-of-atmosphere images.

Its strength varies with atmospheric conditions (e.g. more aerosols or water vapour scatter more light, hence the blue cast will be stronger) and sun angle (lower sun angle will cause more light to be deflected in the direction of the sensor).

The reason the additional light is blue is related to the reason the sky is blue - light with shorter wavelengths is scattered more.