Trees that exhibit reds/oranges in fall do so because of anthocyanin which is produced in the leaves to shade the photosystems. In varieties that produce it, dryness and bright sun tend to enhance red colorations in fall. Further anthocyanin production can be triggered by putting the potted tree in shade for 4-6 weeks before putting it in full sun (one must toy with how much shade is 'shade' and how much sun is 'full sun' because sometimes leaves get sun scalded too).
This works well for most acer palmatum varieties (Orange Dream is a notable exception it just turns yellow and burns up) and superbly with acer circinatum. The effect with my zelkovas is ... meh - they tend to just yellow. It works on stewartia pseudocamellia and probably also monadelphia. Most azaleas will redden too, but it isn't at all interesting (just makes them look as they do in the dead of winter, but leafier).
Yellows arise because of xanthophylls which have a function of protecting the thylakoids from desctructive acidification by diverting photo-energy away from the photosynthetic 'machinery'. Similar shade adaption --> 'full sun' should intensify yellows, but it hasn't had much effect in my playing. It is worth noting, though, that extra xanthophylls is why the needles of alpine conifers yellow during the winter (Japanese white and lodgepole pines are notable examples).
