A brilliant yellow morph of the insular pit viper (Trimeresurus insularis). From a distance the snake almost seemed as if it were green, likely from the sunlight filtering through the trees above. Based on my human visual system I would say this morph is more difficult to discern from background vegetation than the green and blue morphs, and it could be cryptic among the many colors of surrounding tropical vegetation. It would be interesting to understand how their natural predators perceive the conspicuousness of different morphs. The yellow coloration immediately reminded me of the yellow eyelash vipers (Bothriechis schlegelii) I encountered in Costa Rica, though the morphology of Trimeresurus differs substantially from Bothriechis.
Below are some photos of B. schlegelii from the tropical rainforests of Costa Rica: