I have one also in Fairfax. Mine gets red in the summer when it is hot. I don't know if it is the heat or the sun. My guess is that it is the variety of tree. There are some upright varieties that are known to hold their color really well all season, and these are usually propagated by grafting a piece of the desired plant onto a small Japanese Maple grown from seed. Two of the more popular upright varieties that hold their color really well all season are "Bloodgood" and "Oshi Beni". Some varieties lose their color as the growing season goes on.

Not sure that Miracle Grow is the right fertilizer for your tree. In fact, you may be overfertilizing. I would stop by one of our local nurseries-depending on where you live...Greenstreet Gardens in Alex City at the convergence of Quaker Lane, Braddock Road, and King Street, Campbell and Ferrara on Little River Turnpike (got a notice they are moving, but not sure when or where), several down Route 1 south, Behnke's in Beltsville or Potomac (my favorite other than Greenstreet which is so much closer), and Merrifield. Just make sure you talk to one of the tree folks, not a worker bee.

Oh, also, I would just insulate the pot in the winter rather than bring it in the house. Maybe a windbreak as well if you are up high.