So why are most PCBs printed with Green Soldermask ?
There are a few competing theories for why the solder mask of PCB is commonly green.
The US military required PCBs to be green
When mixing the base resin and the hardener together, they turn green
It is an ergonomic choice due to the human eyes ability to detect green, and the contrast of green with white text
Some combination of the above
Liquid Photo Imageable Solder Mask (LPISM) technology was developed in the late 1970s and early 1980s to to meet the new application demands placed upon solder masks by the rise in surface mount technology. It seems that modern, green colored PCBs emerged with this technology, and the technology seems to trace back to this patent from 1980.
Consequently, endeavours have been made to produce improved processes for producing a mask image of relatively high resolution for the small-conductor art. It was therefore a relatively obvious step to use photo processes in association with UV (ultra-violet) sensitive photopolymers.