There were 2 different inks used in practice-Solder mask and legend ink.
The solder mask is used to cover the entire PCB board and is usually green, but it can be almost any color. It is UV activated, so the areas that are masked can be scrubbed off while the UV- exposed areas are permanent.
The legend is not necessary, but nice to have for placing the components later. Legend ink is an epoxy with a very long drying time. Cure it in an oven to quicken the drying time.
Read More: Black PCB Boards and Other PCB Colors