"Red Sea Rig" is technically the 2C order. It calls for elemental short-sleeved shirt (without pins/ribbons) and pants, shoes, and branch cummerbund. I think it states you should wear the same shoulder boards you'd wear in a white mess kit jacket, but "permits" you to wear slip-ons on the off chance you left your custom blue shirt with loops instead of epaulettes at home. It's all in CFP 265.
That said, I've never seen it routinely enforced outside of RCN officers. Even for us, it's largely dead outside of special occasions. One night in ALG, a few of the aircrew came to the evening movie in the lower half of a two-piece flight suit, with tartan cummerbund and blue shirt (wings still attached). I believe they called it Blue Sea Rig, and it looked incredible.