Basically, they take less coats over any undercoat to produce a good coverage.
One use is to provide a good base to put another colour over the top. Mechrite Red looks slightly pink, but a coat of normal red on top of that will give you a solid smooth red starting from a black undercoat. Same with Iyanden Darksun for Yellows.
Other colours like Khemri Brown and Dheneb Stone can be used in their own right as fantastic colours for cloth and such if you want a great beige tone.
Orchide Shade is great for painting orc skin.
Charadon Granite is amazing for any drab militaryish grey/green. Or for basecoating stone. Or also orc skin.
Treat them like you would any other paint really. Add water as with all paints. Even then, they will cover the undercoat in 1 or 2 coats.