You can also take a look at the Vallejo "model colour" range. These have (unlike their game colour range) a more realistic pallet of colours. I use them for example in the red basecolour (Burned cad. Red is the best foundation for red you'll find) but I also like the green colours as these are very realistic military colours.
However, if you do not feel like buying Vallejo colours (which I can understand, but remember they are a lot cheaper. More quantity of paint at a lower price

but then again, the sell those colours at my local gamingstore) there are two ways.
1) the Foundation paints of GW are already more desaturated green. This gives a good start. Especially the Knarloc Green and Orc Hide shade fall into this group. Then follow up with the Camo Green, and Rotting Flesh which both are more greyish.
2) Mix greys into the colours you wanna use. Start this with Codexgrey in the first and slowly mix in Fortessgrey in the lighter colours. The mixing makes it harder though to keep consistent colours thoughout an army (harder at least then using Vallejo colours

) Another way would be to mix in a little rotting flesh into each colour, but this will not give you the 'tank look' you gave in the above picture.