Many people assume that watercolor and gouache can be handled the same way because both are water soluble, and they are compatible in many ways. Actually, watercolor and gouache perform very differently. But once you understand the differences, gouache becomes a joy to use. Combining watercolor and gouache is quite a logical process. Each medium has its own characteristics. Watercolor’s transparency and luminosity are its strength, so it’s best to use watercolor where luminosity is needed–in backgrounds, or skies, or first layers. But if you have used watercolor for very long, you have already discovered that it can be a rather unforgiving medium. Once it has dried, it’s quite difficult to lift off, remove, or change. This is exactly where gouache shines! Because it can be reactivated, it can easily be lifted off or blended into another color or value when a softer edge is needed. In some cases, gouache can be lifted off right down to the white of the paper! Gouache can be used in transparent washes, but because it can be reactivated, its strength lies in its opacity. So using watercolor washes for those pale, transparent, luminous areas and gouache to build up the solid areas right over the dried watercolor makes sense. In this regard, consistency is king–for gouache, use a thicker consistency–think coffee creamer or even soft butter. Use gouache’s velvety consistency and opacity in subsequent layers, but not in the initial transparent layers. So that’s the order of operations–use watercolor for initial 1st layers that you want to remain luminous. Then when that's dry, use gouache in the subsequent layers. Your success in your painting will increase dramatically and you’ll be much more pleased with the results. Enjoy! And until next time, Happy Painting!