Why do my Japanese maples have so much color variation?

Japanese maples have a lot of variation based on their cultivar. As a general rule, Japanese maples change to a more red/purple leaf when they are exposed to more sunlight. The problem for use is the strength of our sunlight's UV means they typically burn. In your case, I would suspect that it's getting more shade thus it needs to improve its photosynthetic efficiency by having a greener colored leaf.