Leaves 🍁 contains different pigments of various colors.
When they’re fully functional 🌿(spring and summer) leaves absorb all colors (red, purple, yellow and blue🐞🐥🦋) but not the green 🐸, so that’s how they look. When they die out, the different pigments are destroyed one after the other. As the different colors are not absorbed anymore, the leaves change appearance 🥀.
The first pigments to be destroyed are generally the ones that absorb red, so leaves 🍁 first turn red, then yellow, before all pigments are deteriorated and they die, turning brown 💀