I have had a few people ask me about how to do this technique. This is an effect I do on a lot of my Tomb Kings and even works well for anything you want to glow green.
It's a pretty easy effect and just takes time and a little patience to get looking right. I usually do this last, so you want to make sure everything else on your model is finished.
The first step is to paint the entire inside of the eye socket Dark Angels Green (Caliban Green in the new colors). Nothing fancy here, just make sure you get a nice even coat of paint the covers the entire inside of the socket.
Next take some Snot Green (Warpstone Glow) and paint a slightly smaller area of space within the socket. You want to focus it towards the center of the eye, but at this point you still want the lighter green to spill out towards the edge a little bit.
Now I take some Scorpion Green (Moot Green) and paint it only in the center. What I mean by the center is the part of the socket that is flat, so it can be fairly large. You may have noticed that the paint looks fairly wet in a lot of the pictures, thats because I make it pretty watery so the color can flow into the space easily and conform to all of the edges.
I then mix some white into the Scorpion Green and paint a small dot/circle in the very center of the eye, this really gives it the glowing effect.
Unfortunately this process isn't very cut and dry, and at this point I went back with all of my colors and adjusted with the size and brightness of the glow until I was happy with it. There aren't really any guidelines for it, and if you were happy with the effect at the end of the last step then you can skip this one entirely. I would say the main trick to use to achieve the effect I obtained on my constructs is to keep the paint thin to help make the transitions between colors smooth. I even did a little wet blending in place while both colors were still wet. Since the eye socket is fairly recessed it takes the paint a little longer then normal to dry so you have plenty of time to do any blending. In fact I ran into the problem of the paint not drying fast enough and had a few of my colors blend more then I wanted to.
You're all done!
You can see in the model above that I extended the glow to around the edges of the eye sockets, this is called Object Source Lighting. I simple took some of the Scorpion Green and watered it down a lot and gradually built up the color around where I wanted the glow.
This Tomb Prince had the same effect done to his mouth.
You can see all the different ways I have applied this technique, and each way varied slightly. Some had a larger glowing area, others had more Scorpion Green and less Dark Angels Green. It really depends on the situation and there is no "right" way to do it, this guide is just the way I painted it for my chariot's constructs.