There are many explanations why PCB Solder Mask were originally green.
One of the reasons was because when the  PCB Solder Mask was first produced, the base resin was a brownish yellow and the hardener was a muddy brown.

When you combined these two components to form the  PCB Solder Mask, it turned green.Another reason was because the color green has been proven by many scientists that the color is the most visible and noticeable to the human eye. Colors are just different wavelengths and yellow and green are the wavelengths that are most easily noticeable under normal light. The color green is the easiest to see, and also easy on the eyes so they won’t tire the human out.In short,green is the best background color for contrast against white silk-screened text on the surface of the PCB in different environmental conditions.This helpful factor helps out assemblers and quality control inspectors so maybe that is why green is the iconic color that the image of a PCB has come to accept.