From Katie Rees
No one seems entirely sure why circuit boards are usually green, but there are lots of theories. Some claim it’s because the materials they are made out of are naturally greenish. However, the plastics used in early circuit boards were actually an ugly brown, and may have been coloured green to make them more appealing. Others suggest green was a military requirement, because components are easy to see on a green background. Although circuit boards are sometimes made in other colours, it seems it became industry standard practice for them to be green, and the tradition continues to this day.
Answered by Tom Lean for Brain Dump, How It Works issue 93