Steel cartridges were either copper or brass coated to prevent rust and extraction problems but later they were given this proofing by either phosphate or by zinc galvanising which left the cartridge case surface with a smooth matt grey colour of varying shades. Other methods were when the case was lacquered with a transparent lacquer ranging in colour from very light brown or light grey to grass green. Cases could also be oil-blackened or parkerised
''Info by Dan Mouritzsen''
Here's a couple of photos I just took of a 7.92mm Kurz round for you. Hopefully I've inserted the pics correctly. It has a steel case with a dark grey green colour and looks to have a lacquer coating on it. I know it's not quite an 88, but it's a late war german steel case, so perhaps the same finish? I should say, I believe it to be german late war. It's definately a Kurz round.