To be honest I normally always overbrush brown - what it does is raise the detail from the black undercoat & let me see what's what so much more clearly; it also allows for a greater shade 'depth' when painting - rather than colours competing against the black undercoat (as I will tend to paint with quite thin paint - at least when I'm not speed-painting!); it also, of course, can save a load of time when horses / muskets etc are involved as it'll give you your base colour to highlight onto.
I decided to try the grey overbrush as an experiment with the Austrian infantry purely because of their white uniform & the decision I'd made to fast-paint these: I wanted to see if this would save me a stage [which it did] by not painting grey onto a brown overbrush before going to white...it worked fine, but there's not quite the depth there that you'd get if you overbrushed brown first & built up from there.
Hope that helps - if you like I can take some close-up 'underneath' shots of some Austrian infantry- let me know
