Personally, I don't bother with undercoating (AKA, Priming) unless the parts are going be handled a LOT, and/or are made of metal. IMHO, it just adds another layer of paint, and once you've applied it, you run the risk of wiping out any delicate detail that you may have to put more than one coloor-coat over. (I'm speaking strictly armor here, as aircraft with natural metal finishes are a different animal)
There ARE some exceptions though.. For instance, if I want to depict a layer of paint that's been exposed via traffic and chipping, then the original color coat goes on, followed by the masking medium (I use rubber cement), then the top color-coat.
For washes, generall speaking, a contrasting color is best, although black is more often than not too intense... Burnt Umber, dark brown, or dark grey is better, IMHO... Save the black for the edges of hatches and such..
In the case of a pin-wash over Panzer Grey though, dark brown or black would work nicely... Darkening the Panzer Grey with black is good too..
Also, keep in mind that washes add depth, so you also need to think about dry-brushing in concert with the wash, which is used to highlight raised details like hinges, tie-downs, edges of fenders, etc.. For that, you'll use a brush that's been dipped into a a light-contrasting color, like adding white to the Panzer grey in small batches, then wiping about 99% of the paint off the bristles, until it's almost dry (hence the name).... Theh you'll drag the brush lightly across the raised details, catching the edges and nuts, handles, latches, etc...
Keep adding white to the grey, each time brushing more lightly until it reaches the level you want.. Rule of thumb generally is, when you get to the point to where you think "one more pass will do it", STOP RIGHT THERE!.
Drybrushing is easy to over-do, and once it's done, it can't be undone, like a wash can... Takes a repaint to un-screw it....
There are a number of primers here, many of them in the Painting Forum...
/search/SearchResults.aspx?q=Washes+AND+sectionid%3a18&o=Relevance
If nothing in those areas help, or you don't understand something, feel free to ask someone.
Also, there are tips here:
http://sheperdpaine.atspace.com/
Shep Paine is one the best painters out there, IMHO... He literally "wrote the book" on weathering, washes, drybrushing, rusting, and of course, dioramas... His two most important books, How to Build Dioramas and Modeling Tanks and Military Vehicles are, if nothing else, the basis for everything one needs to know about the subject...
Guess I'm somewhat of a "Paineiac" (I know.. Groooaaannnn) but that guy's work with Monogram kits back in the 70s was what got me into being a "serious" modeler, and full-blown diorama-freak....
One last thing, make sure that your washes and dry-brushing are done with paints that won't attack the underlying paint either... For instance, if you used enamels to paint the vehicle, use acryllics to do the wash and dry-brushing, and vice-versa...
Lets us know what and how you're doing on your work... Folks here are eager to help..