That's immitation rust. Rust-All can be found in most good hobby shops or a Miro-Mark,com

It's a 4 bottle kit...use a blackwash if desired, paint on some from the rust bottle, may take 2-3 coats to get it where you want...pioneer tools, tow cables etc, one coat should do. Bottle 3 is the small dust granules to spread on muffler shields etc to replicate flaking paint. Last bottle is a tone & gloss flattener. It's great stuff, not expensive and a must for German and any other tank with mufflers or muffler shields. One kit will last a long time.