From an article I wrote for Art Association Napa Valley.
Drying shift can be a challenge for watercolor artists. Many of us experience the problem where watercolor paints shift color or value as they dry. In my sample images above, my dark grey roadbed becomes a paler grey when dry.
The reason has to do with the different refractive indices of water and air. As the water dries and evaporates from the paper, the paint appearance is affected by air more than water. You can read about this effect in detail on the Handprint website written by Bruce MacEvoy.
The important thing to understand is that you need to be more assertive with your color initially so that you end up with what you intend. An additional challenge is that watercolor pigments have differing amounts of drying shift. Some pigments change very little whereas others change great deal.
The data in the Handprint Watercolor Paint Drying Shifts table can be overwhelming, but I recommend picking out a few of your favorite colors to see the drying shifts MacEvoy has recorded. It helps explain how much adjustment you might have to make.
For example, two of my favorites have very different drying shifts. French Ultramarine Blue has a total drying shift of 47%, whereas Hansa yellow has a 14% drying shift. That means I have to make my blue a lot stronger than my yellow to end up with similar strength of each pigment.