Red or "Copper" is high temp. Blue is a general automotive sealer. Alot of the gray silicones are great for putting such things as ATV blocks back together where a gasket isn't used. The gray tends to have excellent adhesive qualities along with the ability to lay on very thin layers. You can actually use RTV clear on your pumpkin cover with no worries. Clear tends to clean off easier the next time you remove your diff cover. For those stubborn to seal spots such as transfer cases; I like the black silicone called "The Right Stuff."