The green that you see is the color of the solder mask that is laid on the topmost layer of the assembly (to prevent short circuits, soldering errors, etc). There doesn't seem to be a clear consensus as to why most solder mask is green. However, here are some of the theories:
Green was the color of the original resins that were melted to create solder mask in the early days of PCB manufacturing.
Green came from some military standard, or military testing that indicated...
Green is the best background color for contrast against white silk-screened text on the surface of the PCB in different environmental conditions.
One is no longer limited to green however, and most PCB manufacturers like to advertise the fact that they can produce boards with all different colored solder masks.
On the other side,The green color came from the military- at the time. There are other colors available. In the military(USN), it was not just the solder masks, but conformal coatings that changed the colors. EDIT: There were purple coatings for submarine PCBs,and there is a reason, but if I tell you, I have to kill you;-). In one
job, the company's marketing colors were blue, so I paid extra for the precise shade of blue for my PCBs.
The color of FR4 (fibreglas) material, commonly used in PCB manufacturing is a weird shade of grey/brown. If it had no solder mask or coating, the end result would be kind of a dull grey/khaki/brown one used to see in certain old TV sets, where the PCB was integrated into the steel vacuum tube mounting frame. These boards did have legends for the components screened onto them, but they were not coated in any fashion.
Conformal coatings came about in the military to provide some protection from moisture and corrosion. Since the military was almost the largest customer, it was easier and cheaper for many companies to provide coatings and silk screenings to all customers. After all, you still had to pay the NRE and material costs of these screenings and coatings, but it was greatly reduced prices than the original prices.
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