Here's how I paint all of the lenses on my T'au!
I use VMC & VGC colors for this, but any set of dark, medium, and light colors should work just as well.
Base the lens with Black Green 
Paint one half of the lens, diagonally, with Sick Green 
Give the Sick Green area of the lens a thick edge-highlight with Scorpy Green 
Finally, use any White to add one reflective dot to the center of the Scorpy Green highlight, and another to the opposite edge. You've now got a lens that looks great on any model!
KirbPaints
I paint T'au & T'au accessories. And other stuff!
Vallejo Game Color
Base 2
- 72.029 Sick Green
- 72.032 Scorpy Green
Vallejo Model Color
Base 2
- 70.951 White
- 70.980 Black Green
