Soldermask makes the PCB green
To protect the conductive pattern from dirt, oxidation, moisture and mechanical damage, and to prevent solder bridges when mounting components, the PCB manufacturer applies soldermask to the PCB and only the soldering and contact surfaces are left exposed.
Using a photo-tool requires the areas where the soldermask is needed to be clear to allow the UV light to pass through which then hardens then ink. Areas where no soldermask is required (component pads, contact areas etc) the photo-tool is black, this prevents the UV light from passing through and thus the soldermask in these areas is not hardened and is removed during the developing process.
The first practical soldermask was green and there was no alternative.
The industry has become accustomed to the green colour of soldermask and it has an advantage. Until the late 1990s, printed circuit boards were controlled exclusively by men. Usually, women had to do the strenuous task of manually inspecting the boards for many hours per day, they did this by eye and had to be able to detect as many faults and anomalies as possible.
It has been known for centuries that our eyes perceive the middle colour spectrum, (yellow and green) most intensively. Green light is easy on the eyes, while blue-violet light damages the retina. VDU workers have known this for a long time and people who wear glasses no longer want to do it without their VDU glasses with blue light filters.
Although solder resist is now available in many other colours (black, red, blue, white and even transparent), green soldermask is still the most widely used colour in PCB production. Not least because the algorithms of the automatic inspection systems that now take over the manual inspection of PCBs are optimised for the green colour. So if you want to be absolutely sure and want the cheapest solution, stick with green PCBs.