Only one color, but not one size, Stuck at the bottom, yet easily flies.
Present in sun, but not in rain,
Doing no harm, and feeling no pain.
What is it?

It's a shadow.

Shadows are black

They are different sizes

Rain makes the sun be blocked, making no shadow

It does no harm and feels no pain unless it's a shadow from The Princess And The Frog

answer; fulfilling a goal of the president's council of physical fitness and sports;

answerstructuralism; functionalism;