Many non-variegated JM's need sun from morning til mid afternoon and then shade to have the best color. Don't transistion them too fast though or the leaves will burn. I usually do a couple weeks shade then dappled morning sun. If they still look good, a couple weeks later I try morning sun only and gradually increase it to my desired amount. The full transition need not be so gradual if your trees start the spring in morning sun but summer is a different animal.

Well I'm happy to report that my Fireglow is now mostly red! It has only been a week in full sun and some of the leaves look like little red stoplights. It definitely has some sunburn, but all in all it is ok. My Shaina is still more green than red, but I expect it to stay that way because it is in mostly shade.

Red-leaved JMs maples will keep their color much better in the sun, but too much sun can often cause leaf burn. The best situation is generally full sun in morning to midday, with light shade in afternoon. I planted my Shaina in a spot that was initially pretty sunny and it kept deep red leaves all summer, but it has become more green as the surrounding trees have grown larger and cast more shade. I will probably move it to a sunnier location next fall so it will keep its red color better.

I have another JM (Garnet) that always has beautiful red leaves in early spring but loses most of its color by late May. So I moved it to a sunnier location last winter, and it has kept its red color much better this year. Garnet is one variety that I think does best in full sun.

Julie,
If a tree starts in full sun in the spring time it gets used to the sun gradually so I think you will see less burn in following years. Typically smaller trees burn easier as well so with age comes hardiness more often than not.