A word about Gouache.
Gouache paint is similar to watercolor painting but modified to make it opaque. A binding agent, usually Gum Arabic is present, just as in watercolor. Gouache differs from watercolor in that the particles are larger, the ratio of pigment to water is much higher, and an additional, inert, white pigment such as chalk may also be present. This makes gouache heavier and more opaque, with greater reflective qualities. Gouache generally dries to a different lightness than it appears when wet (lighter tones generally dry darker, while darker tones tend to dry lighter), which can make it difficult to match colors over multiple painting sessions. Its quick coverage and total hiding power mean that gouache lends itself to more direct painting techniques than watercolor. En plein air paintings take advantage of this.
I try to take advantage of this when I am looking for a certain organic mix of pigment that does not come out of a tube.