I think you would get a good effect using the same method you used for your space hulk marines (using grey rather than red though, obv.

). If that style of painting works for you, keep using it.
It should be relatively easy to adjust the shade of grey you get in the end by simply using black wash. Use it straight from the pot and in large quantities to get a nice dark grey finish, then pick out a few brighter highlights when the wash has dried.
If time is a major constraint but you want to try a different method, pick one model and have a go at blending rather than dry brushing:
Start with a coat of chaos black, then block in all the armour with your preferred tone of grey. give it a thin wash of black, then build up your highlights by adding a little white at a time to your base colour and painting it onto the raised surfaces of the armour. Give the model another thin wash of black to help smooth the blending, pick out the details and you're done!
Painting this way does take more time (I use this method and can happily spend 20 hours painting 5 basic troops) and will look similar to dry brushing at a distance but will look alot better close up.