Citadel Underhive Ash Dry Paint is specially designed to paint miniatures very quickly and easily using a technique called drybrushing. It's really easy to do this, and it looks great because it really makes the details on your thumbnails pop out.
When you open this brown-green paint you will immediately notice that it is very different from normal paint due to the thickness of the paint. For that reason, you don't really need to shake them.
The Underhive Ash Dry paint is best applied with special brushes for dry-brushing, these are much coarser than regular brushes, much stiffer. That is important for this technique. Apply some brown-green paint to your brush and blot it on a piece of kitchen paper (this will absorb more paint than a smooth palette, for example). Then apply it to your thumbnail by swiping the brush back and forth over the detail. You can see right away that the brown-green paint has captured all the raised surfaces in every detail and really picked them out.