Picked up from the Arts & Craft section of my local branch of The Works for a miserly 3 quid, a pack of 12 pastels all in different shades of grey. I'm going to be modelling a large area of tarmac and think that rubbed against some emery paper, the dust should be perfect to add a variety of colours to the base grey paint.
I particularly like the green-grey, brown-grey and blue-grey versions. Turned to dust, they will be ideal as weathering powders.
Available in store or on-line.