I look on wikipedia, I look on other websites. It never ever tells you what atoms Photopsin is made out of (I can't imagine why, if you have an article about a molecule it should say what is in it). Does anyone here know?
Maybe because there are different kinds of photopsin. I see Wikipedia mentions 11-cis-retinal. You might chase that connection. Here's a little more on that. http://www.ch.ic.ac.uk/vchemlib/mim/bristol/retinal/retinal_text.htm
There's an article in SciAm April 2009 about trichromacy and its evolution in primates, if you can get your hands on a copy.