Why is solder mask / resist / overcoat green?

Printed circuit boards (PCB) come in a variety of colors, including green, blue, red, brown, and even purple. The most common is green, but that can vary by application, industry, and the age of the PCB. Fashions change over time, and so do common PCB colors. Today, green is the most common. The reason? Not for any significant performance difference. Arguable, some colors may be easier on the eyes when assembling, soldering and reworking electronics, or work better with inspection optics. In reality, the main reason is probably price. Whatever is most common color will generally have lower cost, because bareboard suppliers are running that color continuously. Any odd color will require a change-over, extra clean-up, and thus additional cost.