What is hue in gemcraft

Imagine a color wheel going from 0 to 359 (like the degrees in a circle). The hue of the game will do less damage to a monster of the same hue (approaching zero if the monster's hue is the same as your gem) to double damage if the hue of gem is the opposite side of the color wheel from the hue of the monster.

If your gem has a hue of 1 and the monster has a hue of 1, your gem does 0 damage.

If your gem has a hue of 1 and the monster has a hue of 181, your gem does double damage.

You can change the hue of your gem by combining it with other gems.

Basically this encourages you to have a variety of gem colors available.