In wartime tanks are painted with whatever paint is available. In combat paint fades and becomes dirty. As such there is a huge variation is colour and no definitive ‘authentic’ colour. Look at my reference pictures of real WW2 armour to see the variation between real tanks. The following are the paint colours I recommend using:
I use Tamiya & Vallejo model air paints to paint my tanks. Their colour ranges are both extremely good. I allow four 10ml tubs of Tamiya paint (diluted 1:3 with suitable thinner) or two 17ml bottles of Vallejo model air to totally cover a tank.
For US & British tanks I favour Tamiya Olive drab (XF-62) or Khaki drab (XF-51). Soviet tanks are best painted with Vallejo model air Russian Green, but Tamiya olive drab, while slightly darker is also very suitable.
Early German armour is best painted with Tamiya German Grey (XF-63) ideally with a dash of Nato Black (XF-69) to darken it. Real panzer grey is almost charcoal grey. Nato Black is also superb for painting tyres, tracks and exhausts.
For spring 1943 onwards German armour was delivered to front line units painted a yellowish-tan colour. I use Tamiya dark yellow (XF-60) or desert yellow (XF-59) with a splash of white (XF-2). Individual units painted their own disruptive patterns of either green (Tamiya olive green XF-58 or Vallejo model air Panzer olive green 1943) and/or red-brown (Tamiya XF-64). Late in 1944 the brown paint factory was destroyed by the RAF and many panzers were forced to use patches of primer red-oxide (Hull red XF-7) to make a disruptive pattern. In combat this quickly faded to a salmon pink or purple colour. This can be recreated by adding a little white to the red. If you don’t weather these colours they can be VERY bright and odd-looking.
In winter crews did the best they could to conceal their vehicles. German & Russian units quickly smeared whitewash over their tanks covering them either completely or in patches. The best way to recreate this on your model is to paint the tank for summer combat, the spray very dilute white so that some of the original colour is visible under the ‘whitewash’.