If the leaves are in the shade, they will be green. If you look at any red leaved maple, 'Garnet' is a good example, the leaves come through a darkish shade of green then progressively get redder the more sunlight they get. Then again, is it grafted onto a different rootstock?, if it is, and the green leaves are below the graft, pinch out, or cut off the growth as it will be the rootstock trying to grow. If that happens, usually you end up losing the top bit to the original rootstock.