Not played The Last of Us then?
Speaking with EDGE magazine, Sony Worldwide Studios president Shuhei Yoshida explained that the bulb changes colour to represent your health in Guerrilla Games’ upcoming first-person shooter. “People were watching the main screen [during the game’s live demo at the PlayStation Meeting], but Steven [ter Heide, game director] was facing the audience, showing how the light bar colour changes as he was hit by the enemy. As he lost hit points, the colour was changing from green to red.”
I'm sure it's not going to be involved in any core gameplay mechanics. Just a pointless little touch that you'll look for a few times in the first month of owning one and then never again. Like SIXAXIS controls.