I start off base coating with an off white color (GW bone white or equal) then I hit it with some white. Once its all dry I hit it with one of my special home brew washes. The wash I used on this one is 2 parts black / 1 part dark brown. Mix the color into a 50/50 mix of water and future floor wax (lemon scented is what I use). There isnt a science to this so its alot of trial and error as far as how much pigment is in the mix. Next time I make a batch I will record the amounts I use.
Once the wash is dry then go back and highlight the very raised edges.
Here are some pics of the interior after it was put together.
I'm not very familiar with terms what is "OTT"?