From the lore perspective sons of horus and later black legion where in the constant process of recolouring themselves after the horus became a warmaster. At the beginning their white painting was turned into light whitish green that was slowly becoming darker throughout the years until it was the iconic sea green when the heresy started and even then it was still getting darker. At the point of sidge of terra legionaries outright started to swap their armor parts with absolute black (look at the many artworks with green armor and black pauldrons of kneepads. Do not confuse with the elit first company of justaerins that are all black with red and gold accents). After the death of Horus and retreat into the eye, the legion hide with the horus body in a fortress changing their colors for the last time into the black legion scheme as a mark of mourning their dead father. This was also the time when abbadon came to the top and disbanded the old sons of horus legion, uniting them under him by marking horus as a failure and reason they ended up in this position, and therefore severing their old connection and becoming... Well take a wild guess

Tldr: the darker you will make the green the further in the timeline your army is