I've seen resources that explained that the color coding on shotshells is for guage, not type. Twelves are red, twenty's are yellow, etc.
Yellow was not widely used for 12ga (red blue and green being most common), so it was a good choice for the 20ga. The main reason was so that 20ga could be identified at a glance.
While it is true that yellow is pretty common for 20 guage and not for 12, the rainbow of colors available for 12 (after looking in my box, 2 different greens, 4 red-burgandy, 2 blues, purple, pinkish purple, black, greyish clear) nope color isn't by gauge.
yes, internally a company may have a set pattern on who gets what, but in general, color will tell you nothing. Add to this that I consider shotgun shells the easiest thing to reload, well, anyone can be sticking anything in those same hulls the second time around.
Some folk will even tell you that you can tell low power target load from full power high in the sky loads by the hight of the brass at the end of the shell, but that doesn't hold true anymore either. Maybe it used to, but it doesn't now