Thanks . I was thinking it must be some kind of coating its rust like in color but it does bot have the texture of rust it is smooth and not flakey at all nothing rubs off on a rag even with brake clean. Kind of wish they had done the inside of the diff that way also it looks bare metalnin there but is rust free. Thanks for complement on the pattern it acutally went surprisingly well soni was worried i did it wrong but it looks ok me. Granted this was done with minimal load my hand and a rag on the ring gear while turning probably more load while checking would have been good but its out of the car on jack stands and not super stable for heavy loads. Well im going to run it ans see how it goes.Textbook job on contact appearance !
That red colour looks like Glyptal Paint. Its common stuff to paint insides of cast iron gear housings and engines with it.