Okey dokey. Here's how I would do it with the colours you have.
Prime it black, then paint all over with Knarloc Green, then lightly drybrush with Knarloc Green mixed with Gretchin Green to highlight.
Then, get Calthan Brown and paint splodges on in whatever way you see fit, vague shapes like this:

I wouldn't add too many colours to the camouflage as it tends to dissapear. Then paint over it again with Khemri Brown mixed with Calthan Brown. (Khemri Brown is very light). Then drybrush lightly with Khemri Brown again.
And don't forget to put some nose art on!

Hope that helps!
SC