What's the difference between red and green? Over the millennia, the an accidental copying of an ancestral red opsin was followed by mutations... some of which shift the absorbance so we can now tell red from green!
Mutations that matter--and don't
Becoming green: Find the three changes are all it takes
Note: some amino acids display incorrect structures because the structure shown is actually COW, not human :-)
Reveals the opsin lysine that is covalently attached to retinal