Photo by Katy Oldham.
Update: My parents found that this mantid's eyes do indeed turn a deep brown in the evening!! This particular individual has staked out a spot in the yard to call home and appears in the same place every day, giving my parents a chance to really observe behaviors and changes. Here are a couple shots of our friend's dusky eyes...
(photo by Nile--eyes darkening)
(photo by Katy--this one's a little blurry due to low light, but you can really see how dark those eyes get)