There are several potential reasons for observing differences in the images.
First, most likely you are observing differences in the level of atmospheric aerosols which causes scattering. To rectify this, you need to use the Sentinel-2 Sen2Cor processor to convert top-of-atmosphere (TOA) reflectance (Level 1C) to bottom-of-atmosphere (BOA) reflectance (Level 2A).
Second, if the scene is from a different time period, there may be differences in the amount of green vegetation or snow--thus producing visual differences.
Third, you may need to adjust the histogram so that the image renders properly. you can do this directly in the Sentinel-2 SNAP viewer. Adjust the slider so that the majority of your image pixel values are encompassed in the interval for all the bands. In this case, you would want to move the upper slider to the left to "clip" the histogram.