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In the past 2 months I've been on fall trips in the Colorado high country and Utah, and I just came back from a trip to Acadia National Park in Maine. I missed peak color in Acadia (I'm guessing by about 1-2 weeks) but despite that I was still blown away by the fall color - most of the red had become a shade of rust and there was also lots of orange, some yellow still even. For a real variety of color, I'd say probably Acadia would be a great spot. The whole park was extremely photogenic. Probably one of my favorite parts were the carriage roads which had masses of fallen leaves strewn on them - that was awesome! Acadia is also supposed to get a lot of fog, but I didn't see any fog until the last 2 days I was there. Would've been nice to have gotten more.
I don't think you could go wrong between Colorado or the Northeast though. Henry W's post sums it up well - CO is chock full of golden trees, the Northeast has a great variety with intense reds.
CO has high mountain peaks (usually snow-covered) with alpine lakes & streams - overall a great area for getting wide-open shots of tree groves fronted by other groves, with a lake in the foreground, and a peak (or range) in the background. Google "Maroon Bells" for a quintessential shot of what Colorado is like.
I really enjoyed my trip to Maine though, and it was awesome to see lots of reds, especially with the timing right before Halloween - there were lots of fallen leaves on the ground and roads, and lots of homes had pumpkins sitting on their front porches. It was extremely picturesque, I snapped lots of pics of houses too.