The coulors seen in the picture match the position and the ammount of the stress in the object. Basicaly the bigger the gradient is, there is a higher tension in the glass, that means it is more likely to break when used, beacause the stress is relaesed when the object breaks. These photos were made as a proof of concept using polarised light from a LCD screen in combination with another circular polarisation filter on the camera. As you can see, it works well.