The earth's axis at the same time of year the northern hemisphere receives both light. Making summer way hotter then the rest because it is closer to the sun.
Due to the tilt in the earths axis at this time of year the northern hemisphere receives both less light and a slightly weaker light from the sun. This causes the chlorophyll to stop causing most plants to stop being green and go into a hibernation state. The beautiful colors are the trees losing the excess parts it can no longer support