No code however
Most North American manufacturers make their 20 gauge with yellow hulls to distinguish them from 12 gauge to help prevent a 12/20 burst. This is when a 20 is put into a 12 gauge and slips into the forcing cone. When the gun doesn't fire, the shooter puts in a 12 and then kaboom. I came very, very close to one of these once and the thought still gives me the shivers.
Currently in my assortment of 12 gauge ammo, I have shells that are light gray, orange, dark gray, bright red, dark red, dark blue, light blue, green, gold, off white, transparent and black. All the 20 however is bright yellow and is kept well away from the 12 gauge. The only 20 I've seen that is not yellow was made in Europe.