you grind pigments with distilled water into a paste, you can store them like that in jars.when painting you will need to prepare paints by mixing pigment paste with water (just as much as you would use in a painting day) and adding a little lime for consistency (half a teaspoon of pigment paste + coffee bean of lime).
Lime is also used as white pigment for fresco.
grinding pigments article:
http://www.truefresco.com/frescoshop/article_info.php?articles_id=4
fresco pigments set:
http://www.truefresco.com/frescoshop/product_info.php?products_id=36