A PCB's colour is given by the solder mask coating and actually PCB's can use one of many colours these days.  The reason green is still often used is probably a combination of its always been down that way and because green solder mask is actually a very good balance of being OK on the eyes for assemblers and effective to look through when the tracks on the PCB need to be seen,  As for why green was originally used there seems to be many unsubstantiated reasons but this is the most likely we've come across:- The original masks used a base resin that was a brownish yellow and a hardener that was a deeper muddy brown. When they were mixed together they created a honey brown color that was apparently not very appetizing. They tried adding red pigments but it became a rusty adobe color and using blue simply made it a darker brown. None of them were very appealing colors. Since the laminate materials at the time had a green hue they tried adding more yellow and some blue and ended up with an acceptable green color. It became the standard color we are still using today.