It goes white to indicate the "active" stage of the tether. Fill the enemy's bar until it's white and fill a crystal's bar until it's white and the two wil then tether and cause them to fly towards each other.
Understand the confusion - we are used to seeing enemy bars go down when we shoot them! But as the player could not hurt the enemies directly I was attempting to show a powering up state rather than an inflicting damage state.