Gouache paint is a watercolour paint which also contains white pigment. It is opaque and can be painted thickly or thinly and is often use in conjunction with watercolour paint.
Wikipedia says "Gouache is a type of paint consisting of pigment suspended in water. A binding agent, usually gum arabic, is also 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 is also present. This makes gouache heavier and more opaque, with greater reflective qualities. It generally dries to a different value 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. This, combined with its quick coverage and total hiding power, mean that gouache lends itself to more immediate techniques than watercolor
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