The reason that in the spring and summer, the leaves are green and clean (Heh, Heh,Heh) because sun is acting as the leaves' food, so in the fall, the leaves don't get the food they need, so they lose some pigments (wich is kinda like protection or the boss for the business for color), wich changes the color ofthe leaves.
When winter comes, the leaves have to eat there waste ( poo and pee for the human) and causes them to weaken up, causing them to fall.
There are lots of ways that leaves change there color, and why they fall.
I hope you learned something (at least from my rediculously long and awful speech)!!!! :)