In various places I have worked we used Red color soldermask to indicate that the PCBAs were in early prototype stages(EVTs,DVTs) and once the final designs wer

In various places I have worked we used Red color soldermask to indicate that the PCBAs were in early prototype stages(EVTs,DVTs) and once the final designs were released for manufacture(PVT) they were Green in color. This also was used to indicate that these boards were RoHS compliant. Again, this might have been very industry specific(Medical devices in my case) and YMMV. IN another FAANG place that I worked our ODMs who also worked with Chinese Fab houses started out with Green soldermask. Nowadays I find it fascianting that one can order preactically any solder mask color one chooses to, though i have a special affinity towards pink.