I paint roofs with acrylics, using a basic grey mix to which I add differing small amounts of blue (or green, depending on where your slates have come from) as I go along, and painting in 'patches'. This gives a subtle variation in the shade without being too obvious or creating sharp divisions in colour. See the roof of my recently completed signal box. If any of the paper slates are tending to lift, you can work some of the acrylic paint under them and stick them down.
Jim