Hello,
Yes Christian is right. Just like a humans eyes, when you are put in a dark room your pupils enlarge then when put in a light room they get back to normal size, except in mantids when dark, the eyes get darker. However, i notice humidity affects the eye color. Not from black to regular but in other cryptic ways, for example, when hymenopus are normal and their eyes are light, when you mist them and the humidity is above average, their eyes will stay white but some violet striped will be going through their eyes.
Thanks
Eros