Rustic tones in modern interior designs consist of stone or browny natural tones. With little hints of *aged* timber, the rustic look can be acheived very easily in even the most modern of homes.
Oh, you meant goblins...
Not sure how to make a goblin look 'rustic', but I can make one look like he is wearing battered and aged armour that has seen many years of disuse...
For my (dis)honourable goblins, I paint their armour tin bitz, then drybrush it with boltgun metal. After that I was the armour with a watered down black ink. After that I wash the armour plates again (not the chainmail), but in patches with black ink watered down again (leaving the edges bare metal). I then think about where would be the easiest places for rust to form on the armour and put patches of Devlan Mud over these areas. Don't worry if it looks too patchy, as after this, I lightly drybrush the armour and chainmail with boltgun metal (really quite lightly, so it is literally the edges) and then for areas which get bashed around the most (edges of shields, swords etc) I drybrush or highlight with Chainmail.
I have painted 36 goblins like this so far and I really like the look of them.