I use this reference article for many tips, washing as well.
http://www.scaleworkshop.com/workshop/paintingsmallscalearmorbg_1.htm
The key is the colour + how much to use.
For instance, I used Sepia acrylic based colour on my leopard 2a5,using brush on panel lines, bolts, all the finer details on the kit. Then I used a very fine spray of Sepia over whole model to finish. Dont be over zealous, be subtle with application, you dont want to 'darken' the model to much. Especially with panzers, base colour might be a desert colour say, a light mix of burnt umber perhaps, just keep checking on that piece of blank white paper first, before any application to model, that way you can pre-determine what the wash colour looks like.
regards
Dave