When you picture a circuit board, you probably imagine them to be a greenish hue of some kind and you'd be right, for the most part, but why are they actually green to begin with?
Back in the 1970s, when the personal computer was in its infancy, circuit boards were mostly a revolting reddish-brown colour. It wasn't until the early 1980s where a new technology called the circuit board Solder Mask was introduced which gave circuit boards a corrosion-free coating, which proved to be vitally important in electronics. The Solder Mask contains glass epoxy which naturally contains a green tint.
Over the years, green became the standard colour in PCB circuit boards, but even more green tint was added into the manufacturing process but for another reason. A large amount of time is spent by technicians and engineers on circuit boards, and studies have shown that the colour green is not as fatiguing on our eyes in comparison to some other colours such as yellow and red.
Nowadays, you can actually get circuit boards in a whole array of colours that are more aesthetically pleasing, mostly in gaming, but remain green in most other electronics.
Take a look at the video below by the YouTube channel: Tech Quickie on Why Are Circuit Boards Green?