Paint on. I watched a Doog's Models video on pre-shading beneath GSB, and got some decent results - doesn't show up as well in pics, but I'll use it again. Used MRP Eggplant, Tamiya Hull Red, Dark Gray, and then Nato Black to blend it in, then the top color coat (a mix of Tamiya paint). I didn't get quite as much contrast on the two colors of blue on the back fuselage as I intended, but will leave it as is.
2 attached images. Click to enlarge.