Why do leaves change color?

I have been thinking about this question a lot in the last few weeks. Some say it's the days getting shorter (less light) and others say it's fall temperatures dropping. The answer is that it's a combination.

This is in Provo Canyon last year. The colors were fiery and vibrant. This year (on our recent trip to Park City) I was a little disappointed that show was not so showy. This website infers that it's the lack of cooler temperatures this fall (since it has been almost 90 until yesterday!) that have caused the colors to be less intense.

So now you know.