Q.What could have caused a red maple seedling to turn green?
The little tree planted itself in my flower bed from my neighbor’s large, old red maple. After about three years, it was about three feet tall. In the spring I transplanted it to the front yard in a mostly sunny spot. It grew O.K., but it’s green! and now its leaves are turning gold in the fall. What happened?

Certified GKH Gardening Expert
It was likely dropped from a different maple, or perhaps the red maple in the yard was pollinated with a nearby maple.
Even if it were self pollinated, the genome of a maple tree is so large and complex that it could never produce an exact copy of the tree from seed.
If you want an exact replica of that tree, you will need to take a cutting and root it.
This article will help: https://www.gardeningknowhow.com/ornamental/trees/maple/growing-maple-trees.htm