My knowledge of red diesel is a bit dated now but it was the same as road diesel except for the red dye which helps Customs to identify it (no special road duty on it) and road diesel has additives which help protect the engine. A red diesel can have a high sulphur content which is most unkind to the pistons and bores of the infernal combustion engine so an aditive is useful to provide additional lubrication. The exhaust emmisions also tend to be smokier and smellier than road diesel because of the impurities and lack of additives.
Oh and it isn't a substitute for avgas, diesel requires a completely different sort of engine.