It’s very simple – dark color on top and light on the bottom of eye and small white dot at the end. It works with any color :) See example below.

Here is set of paint which I use to paint Arachnarok’s eyes.

First basecoat from Orkhide Shade (GW).

Next is Gnarls Green (P3).

Highlight with Wurm Green (P3).

More highlights with Livery Green (VMC).

White dots, and eyes are ready. Later, after I paint whole model I varnished it with Matt Cote and repainted eyes with gloss varnish to get more realistic look.