I'm currently on a bit of a mission doing a load of German units - the reason why - got an airbrush, my paintings improved and I hate a 'piecemeal army'. What do I mean by piecemeal - well it's an army without cohesion, because personally that's what makes a collection of models an army to me.
The issue though with my Germans (and other armies in the past) is that because I do improve over time, I have to almost deliberately paint worse than previous to do a new unit to match a previous army! My Germans are a prime example of this (especially the infantry) I can paint winter infantry significantly better now - having done two whole armies, but I need to ensure they match the old Volksgrenadier models I did. Alternatively I do what I did with my British armour, the first ones I did for my Canadians were very very simple. They were not up to scratch when I came to redo my Brits, so I just restarted - much the same as I'm now doing with my German tanks.
I know some others have alternative ways to tie armies together such as bases or using just the same limited pallet of colours. Sadly with tanks you have no bases, and with regards colours - well they're all pretty similar in WW2!
So I think I'm following Winner Dave down the path of madness, strip and repaint to get that cohesive look - it's going to be a long few months....