As I understand the reason that these first round sith kept with traditionally jedi colours is because they both had no time to change them and they didnt care.
The traditional red saber probably didnt come until the more united sith closer to revan's time
Savior, conqueror, hero, villain. You are all things Revan... and yet you are nothing. In the end you belong to neither the light nor the darkness. You will forever stand alone."