What color to use for a panel line wash

Im building the 1/72 Academy Ju-87G For a riend of mine. I painted it in splinter camo and Im wondering what color to do the panel line wash in. Any suggestions?

Black for the upper colours dark grey for the underside.

Only you can really decide what looks best to you, and since you are building it for someone else, use a paint mule and try different colors and see what looks best to you. To me paint mules are an indespensable tool to have available and I use them all the time. I’m using a couple now to practice painting insignia as opposed to decals. Good luck with your decision.