You could copy the part where it draws the health bar and place the original code in an IF health >= 11, and the copied code underneath it inside the OTHERWISE block and change the colour in that code to red.
again, this is a simple idea, but there might be a more efficient way like making a colour attribute attribute and create an event like this:
IF health >= 11, set (colour attribute) to green.
OTHERWISE, set (colour attribute) to red.
But I'm not sure if that is even possible.
I'm just growing these ideas around from my phone without having access to stencyl, so no succes guaranteed =P