The circuit board can be made in any color you like...almost any color.
The actual circuit board is not green, but a printed material called solder mask. This is a layer of solder-resistant resin, the part printed on the pcb prototype circuit board that should not collect solder. This prevents the formation of solder bridges between the IC pins, and in the old age of wave soldering, it prevents the circuit board from picking up a large amount of unwanted solder.
Green is a pigment in resin formulations. Green is specially selected as an auxiliary method for human body inspection of PC boards. Although most inspections are now done automatically, there are still times when manual inspections are required. Since it is a standard configuration, the performance of the green solder mask is usually slightly higher than other colors. This can be a factor in a very dense design, but for most PCBs, green is just the default setting: unless you order another product, it will get it.
The green you see is the color of the solder mask, which is on the top layer of the component (to prevent short circuits, soldering errors, etc.). There seems to be no clear consensus on why most solder masks are green. However, here are some theories:
Green is the color of the original resin, which is melted in the early stages of PCB manufacturing to form a solder mask.
The green color comes from some military standards or military tests show that...
Green is the best background color to contrast with the white screen printed text on the PCB surface under different environmental conditions.
However, one is no longer limited to green, and most PCB manufacturer like to advertise the fact that they can produce boards with solder masks of all different colors.