Confusing Lightening of Plant

These two pics are of two plants that are the same strain, same age, same container, same distance to light, same ferts and everything is equal because they are in the same container. You can't tell real well by the pics but one is very light green. I checked underneath the cover and they seem to be getting the same amount of spray. They are same distance from pump. What are your ideas as to why it is getting lighter and lighter. It seems to have stopped but I am worried it will stop growing. This is 12 days growth. So they are growing well.