What is the best color for a PPG signal (photoplethysmography)?
The reason red and infra-red are used is the difference in absorbance at those two frequencies between oxygenated and unoxygenated hemoglobin. Using the difference in these two frequencies allows the concentration of oxyhemoglobin to be calculated. (The red LED is at the frequency where oxyhemoglobin and hemoglobin have identical absorbance).

As far as I know, there's no reason why a green LED wouldn't work. You would only be able to measure the PPG signal and not the oximetry data (unless you used two different LEDs and did some math). 

Absorption curves.