Here is a list of the various Herb combinations that you can make:

Two-Herb Mixes:
Green + Green = Restores 50% of total health.
Green + Red = Completely replenishes health.
Green + Blue = Restores 25% of total health and eliminates poisons in the body.

Three-Herb Mixes:
Green + Green + Green = Completely replenishes health.
Green + Green + Blue = Restores 50% of total health and eliminates poisons in the body.
Green + Red + Blue = Brings your character to perfect health.