Code Improvements... and Difficulties

I have worked hard on applying a gradient fill to the mountain shape... what I have been trying to do is graduate from a random colour value to black... similar to Etienne Saint Amant work that is light at the top moving to dark at the bottom.
I have however been having a lot of trouble with this as using a random value in a for() structure keeps producing different colour values in each polygon that makes up the mountain.

Because I have had difficulty with this and found the previous code over-done I am considering reverting back to grey-scale and returning to a sketchbook aesthetic.