Highlighting
Those bits that I had not touched up with white I worked on with a grey wash (Vallejo Model Wash for dark & grey vehicles (76516 Grey)). Perhaps that could've been applied on the off-white bits too, but I'd test that out on the underbelly first, because if that wash ruined the white it'd still be easily fixable - or kept on on the potentially superdirty underside of the shuttle.I began with the most unnoticeable part, the front bottom section of the cockpit module. Then I went through the whole module pretty rapidly, concentrating on the funkiest surfaces and details. Some kind of a brighter spot of colour (red, green, blue on a cable, a button) might improve the area by breaking up the slightly monotone grey world. Perhaps, perhaps.
The front-facing hull parts of the shuttle were going to be mostly overshadowed by the cockpit module, but I knew that the washing would still improve it greatly. Maybe I could've even used brown or even black wash on some areas, but I felt that the grey wash worked just fine.
I had not touched the inside of the shuttle after all, as I was going to glue the ramp in the closed position when I got that far in the reinstallation. The ramp just didn't look good enough to be kept open anymore.
On the rear side the general look with the twin laser cannon was a bit better after a wash, but I couldn't help thinking that maybe the cable-like things might work better in black. Just like on the underside of the cockpit module, a few monotony-breaking details could bring a great deal of improvement with little effort. Those laser gun barrels I'd most likely paint black, because gunmetal (not to mention brighter steel or even chrome) would not quite sit well there, in my imagination.
To summarize: within a few process steps I might add a couple of silly spots of colour to break the grey overall look. The wings were ok so I wasn't going to do anything to them, unless something required a new quick blast of off-white. This'd get done at the same time with the engine cowling, that, as the photo showed, was still grey and unfinished.