Like koiboy said, I would not worry too much.
My "Acer Palmatum Disectum garnet" went from deep-red/purple into light-green with some red and brownish (dried) edges.
This happened in July ! The timing is different every year. This is rather early but I am glad the leafs are still on.
So don't worry, be happy,
All true Japanese maples are variants of Acer palmatum. You have purchased a great tree but may have managed it incorrectly - hopefully not beyond recovery. Japanese Maples come in several leaf colors and forms. The easiest trees to propagate are the green-leaved varieties. So nurseries grow green root stock and graft red-leaved stock off that root.
You have allowed the green stock tree (which will sucker from below the graft) to become the dominant foliage. Your photo shows a classic example of this.
Manage only the foliage above the graft line (if you have any left) and remove all green sprouts below. The tree's symmetry may be upset for a while. Japanese maple is a slow grower and you must give it time to develop its natural form. Heavy pruning is not necessary nor recommended.
Hi, You have allowed the green stock tree (which will sucker from below the graft) to become the dominant foliage. Your photo shows a classic example of this.
where's the photo ? I cannot see one.
anyone who never made a mistake never made anything.