First, I use the details on the tanks as a chance to apply some contrasting colors to the overall scheme. One of the easiest ways to do this is painting rusted metal effects on exhaust pipes, damaged areas of the hull, or anywhere else that metal may have been exposed to moisture. The best color to use for this is some sort of reddish-brown color. Battlefront recommends using Cavalry Brown, but I tend to mix up a slightly darker version of that:
Rear of a Sturmtiger.
Front of a King Tiger. The models comes with battle damage already modeled.
The Yellow-Brown-Green color scheme needs to be broken up by something, and the rust color shown above is a great option. In the above pictures it seems to contrast too much, but that will be mitigated with an ink wash later.
Tools are another area where we can add subtle contrast. Supplies haphazardly strapped to a tank lend a battle-worn feel to the vehicle. For the Hinterhalt camo scheme featured here I chose to use a tan color that contrasts very well, but is simultaneously realistic:
One of my favorite detailing techniques is used for metal surfaces on the tank, such as vehicle machine guns, wires, antennae, etc. Start by using a thin brush to paint the metal black (if it isn't already painted black):
Then, AFTER the ink wash has been applied, take a number 2 pencil and gently graze the graphite over the surface of the subject area. When you are painting features such as a linked chain it gives a very nice result:
Better go back and fix that one rust effect.
A couple more examples of using contrasting colors on the details. Many of Battlefront's tanks, notably the Panthers, come modeled with shell casings. Use a deep brass color to give them a realistic but noticeable touch:
Many tanks also come modeled with artillery poles that were used to level big guns and tanks (don't ask me how, that's just what Wikipedia said they were used for). In real life they had a striking white and red color scheme. Drybrush these poles white, then apply a thicker coat of red intermittently across each stick:
Rare is the tank that arrives on the battlefield squeaky clean. Even outside of combat the tanks are rolling through the mud and dirt to arrive at their destination. These effects are easy to achieve using one of two different methods, both of which are applied after the ink wash. The first, and easiest, is to simply use a "heavy" drybrush of whatever mud color you'd like to apply:
Be sure to get all around the tracks, mud flaps, some of the hull...
The other option is to take a pigment of your choosing and use an old brush to spread it around the areas you'd like to reach. I have found that smearing the pigment with a finger after initially applied with a brush can also work well:
Two different pigments in this case: one for rust on the tracks, one for the mud
Mud pigment on the tracks and flaps
Pigments can definitely be cool, but for simplicity's sake I generally just use a heavy drybrush of my preferred color. If you go the pigment route I recommend making it your last step before applying matte varnish, which is needed to seal the pigment to the tank.
That's about it for now guys. Just FYI I've started a fledgling twitter account @piflamesofwar, it won't hurt too much to follow, would it?