Actually very easy. It's just a basecoat with a wash.
It might even be a "dip" job, where the model is just dipped into a wash agent, and then left to dry.
The only real trick is getting the right formula for your dip. You can probably google "dip tyranid" and get a lot of info.
Automatically Appended Next Post: Here:
http://www.dakkadakka.com/wiki/en/Dipping_Tyranids
That picture you have is a HEAVY dip.