Lerp between 3 colors based on energy amount.

You just need to split it up into 2 different Lerps:

 if (health < .5) { 
     blendColor = Color.Lerp (Color.red, Color.yellow, health*2);
     } else {
     blendColor = Color.Lerp (Color.yellow, Color.green, (health-.5)*2);
     }

The basic pseudo code would be:

 var maxEnergy : flaot;
 var energy : float;
 var greenColor : Color;
 var redColor : Color;
 
 Color.Lerp(redColor, greenColor, energy /maxEnergy);

Though you will need to put the last line in a method, presumably the Update function.

Scribe

EDIT:

just reread your question and completely missed the 3rd color part, it was 1 in the morning :S

@Kiloblargh's answer is perfect (you could replace health with the energy/maxEnergy)

Sorry about that!