I think Abby and iLia mentioned storing pigments for initial use under a layer of water.
I'd like to add to this. I was taught in Florence the art of fresco. As I'm sure you all know, the calcium in the lime paste is the binding agent of fresco painting, just as oil is the binder in oil painting.
(This is why marble dust,which is full of calcium is good to mix into intonaco)

I was taught to take just a scoop of lime paste and set it in a full bucket of water. The water will turn milky white at first but the lime will quickly settle to the bottom. In not more than a day, the surface of the water will develope a hazy crystal film... like ice on a pond in winter... this means that the calcium in the lime paste has spread throughout the water in the bucket.... now here's the payoff....

Take a small jar, break the crystal surface of the water and scoop up the calcium water... it will look like ordinary water (you can take some of the crystal for good measure, but it's not essential).
Scoop your dry pigment into your jar of calcium water and let it sit for 24 hours. This is a great way to help permeate your pigments with calcium, so that when they hit the wet lime plaster, they are all the more ready to bind. Every little bit helps in fresco.

When you paint, scoop the paint out of the bottom of the jar in paste form and go to it! This is great for a project under way, otherwise store pigments dry for the long term.

Nathan