Phil,
Some comments:
1. First, darker is not the same as colder. If they are breaking dormancy, it's because they're too warm. Moving them to a darker spot that is the same temperature will not slow this down, but will actually make the problem worse because any leaves that form will be totally unprepared for the sun when the time comes, due to the darkness.
2. Once the leaves start to sprout, if you put them outdoors, any temperatures near or below freezing will kill any leave that have formed. So I wouldn't recommend putting them outside until danger of frost has passed.
3. What most people think is a "sunny room" is in fact very dim when compared to hours of direct outdoor sunlight. A grow light would be better than a sunny room or window, but maybe you don't have any. Either way, it's still much much less bright than outdoor conditions, so the tree will need a transition period or the leaves will get scorched. Even a shaded spot outdoors is likely much brighter than a sunny room or even a grow light (unless it's a near professional setup). So if you don't have anywhere to put them that is less than 45 degrees, you can do as Jerry suggests and progressively put them in brighter areas outdoors when the time comes.
4. Even if the leaves get scorched when you move them outdoors, it's probably not the end of the world. You should try to avoid it, but if a few get a bit scorched, don't panic. The tree will likely recover fully in time. Now if it fully leafs out with dozens of leaves, and you stick it in direct sunlight and they all get totally scorched and fall off, that's bad. Just do a decent job and you'll be OK.