oneounce, maybe all new 20's are yellow, but I reloaded, I believe westerns, that were red. Color is never a safe way to tell the gauge.
A 16 gauge in the chamber shouldn't allow a 12 ga. to chamber. but i'm unfamilar with 3 1/2" guns.
I happen to catch the guns & ammo episode when they tried to show the damge caused by the 12/20 burst, in a new super nova, it didn't blow up, just sent the 20 down range.