The reason for this is how SNap generates the images. The colour stretching depends on the scene content. On an image which has less clouds it would look hazy.
You can overcome this by adding an additional step.
You can limit the value range of th RGB channels.
red = virtual_red > 6 ? 6 : virtual_red < 4 ? 4 : virtual_red
green = virtual_green > 6.4 ? 6.4 : virtual_green < 4.1 ? 4.1 : virtual_green
blue = virtual_blue > 6.6 ? 6.6 : virtual_blue < 4.7 ? 4.7 : virtual_blue
The virtual bands are the results of the tristimulus expressions.
This will reduce the effect and images are comparable.
You might need to play around with the limits to get a good result.
This thread might also be of interest for you.