And finally, the dorsal view. I saw Bob's recent post of this species and remembered this individual. Many months after photographing this individual, I read an account that may explain why the coloration varies from green to black.

Hogue describes the process in a related species, Dynastes hercules, in Latin American Insects and Entomology (1). I'm assuming the process applies to our species as well. The color may change to and from black within a few minutes and seems to be controlled by humidity. There's also a suggestion that the beetle may have some control over it. A layer beneath the outer cuticle of the elytra reflects yellow (for that species) when dry. When saturated, it permits the black underlayers to show through.

His reference:
Hinton, H. E., and G. M. Jarman. 1973. Physiological colour change in the elytra of the Hercules beetle, Dynastes hercules. J. Ins. Physiol. 19:533-549.