What percent of health does each color correspond to?

Question

What range of health makes the health bar red, yellow or green?

Answer 1

From Generation II onwards, if a Pokémon has more than half of its max HP, its HP bar remains green. If the Pokémon has between one-fifth and half of its HP, the bar will turn yellow (in Generation II, a Pokémon revived to exactly half of their HP will have a green HP bar). If a Pokémon has less than one-fifth of its HP remaining, the bar will turn red

Source

For percentage wise:
Green HP Bar: >51% HP
Yellow HP Bar: between 20% and 50% HP
Red HP Bar: <20% HP

Hope this helps.