Yellow Heet and Red Heet Fuel additives are pretty similar except for the ingredients being a bit different between the two.
Yellow Heet contains methanol while Red Heet contains what's known as rubbing alcohol.
Also the Red Heet fuel additive can be used in all types of engines and fuel systems including diesel engines and 2 stroke engines.
As for the Yellow Heet fuel additive it is not to be used in diesel or 2 stroke engines as it can damage the engine and fuel system.
So if you have a gasoline engine you can use either the Red Heet or Yellow Heet and both will work just fine.
If you have a diesel engine or 2 stroke engine then you should only use the Yellow Heet Fuel additive to avoid damage to your engine and fuel system.