To understand why trees are green is to understand light.

Trees are green obviously because of chlorophyll.

A leaf is green why? Because it has no use for green light because there is more energy in the rest of the suns spectrum .

As the leaf dies it no longer wastes energy of chlorophyll and withdraws it to save its resources for spring.

Death of anything including a leaf is something natural selection cannot evolve from so it’s just down to which molecules are still present and how they react to the light . Light/frequency is interlinked with all the elements and they either reflect or absorb this light accordingly so yes it’s random but perhaps this specific leaf has a more complex chemistry.

So to get to the point the yellow and red now means the leaf is now reflecting the high energy part of the light so it’s basically dead . Remembering that the Color of anything is only that way because it’s reflecting and nowt absorbing that Color .

Understanding light gives more answers to more fundamental questions than just the way trees use energy.