I got the simple idea to represent the unit life percent by a color (green -> yellow -> red). But the problem was how to keep the texture of my unit and to add the possibility to see the life color ?
I could have used a simple sphere or bar close to the unit but I didn't really liked it. I also tried to colorize the unit but the result is not really beautiful.

One of my idea was to draw the edge of the Texture with the color. And one of the easiest (and also fastest) way to do that is to use Shaders. I'll now describe those simple Shaders in GLSL