click on Details and Options.
which says Mathematica will automatically take the max and min values in your array and scale them so they lie from 0. to 1.
That is why when you multiply your whole array by 100 the result is unchanged, Mathematica scales to give exactly the same result as before.
If you do instead
and you make all your elements in your array between 0. and 1. yourself then you can control the darkness of each square.
If you need more help then please attach a small notebook with a small array which you have evaluated and carefully describe how you need the graphic to be different.