What colour to use for a wash

i'd use a burnt sienna oil wash.  lamp black is another option but tends to be "too black" and therefore unrealistic, unless you're using it on some sort of engine screen where it truly is really dark inside.  it's also good if you need to show oil marks and streaks.  don't forget to highlight panels with lightened olive drab (mix with small amount of yellow) as this helps the base color "pop", sort of the opposite of a wash.

Thanks for the advice this all good least I've got something to go on  now thanks.

I would wash it in medium and dark browns, burnt umber and the like.  Just a thought, add a dash of yellow to your olive drab, lighten in a bit before you weather or it may end up too dark.

Chris