I had a bit of time so took a break from work and spent half an hour in the garden; they were basecoated with Wilco black satin hobby paint (surprisingly good and 1/3rd the price of GW spray cans).
Then Army Painter armour grey
Then Tamiya Panzer grey
Then Tamiya gunship grey
Lastly Tamiya ocean grey
So they fit the theme fairly well now.
They need the obligatory glowing orange eyes
And (hopefully subtle) washes of various colours for tone, contrast and visual interest.
Finally some actual detailing for particular items like smoke grenades and flamer nozzles.