Gouache paint is a watercolour paint with chalk added, which allows the painter to create bolder colours than watercolour and achieve near-complete coverage by painting one colour over the other. Gouache behaves very similarly to watercolour paint, including the fact that it "re-wets," meaning that paints can be mixed directly on the paper even after the first layer has dried. Painters can also mix gouache colours in a palette, then apply the mixed colours to the paper. These attributes give gouache paints a versatility that other paints, such as acrylics, lack. Because gouache will continue to re-wet for years after application to a palette, artists can mix colour combinations on palettes that they can use for years to come.
Moisten your brush and load it with paint. Paint directly on top of a dried section of gouache to mix the two colours directly onto the paper.
Spray a section of your painting with a light mist to re-wet the colours. You can now add paint directly to the painting to mix the colours you've already applied with new colours.
Wipe sections of the painting with a damp paper towel to blend the colours, soften the edges and reveal the lower layers of colour that you've applied. Wiping too hard will cause the colours to appear smudged rather than blending or mixing. A moistened cotton swab accomplishes the same goal when working in smaller areas.
Squeeze out a dollop of colour on one side of a plastic palette, and squeeze out the complimentary colour on the other. Complimentary colours are red and green, yellow and purple and blue and orange.
Pull each colour to the centre of the palette, using a palette knife, and mix them in the middle. At the very centre of the palette, the colour should appear very dark, with increasingly lighter shades as you move toward the colours at either end.
Add dollops of white paint at the edges to lighten the intensity of the colours in the palette. Add dollops of other colours to add tints to the two complementary colours you've chosen.
Gouache paint has both a masstone -- the pigment used primarily in creating the paint -- and an undertone or bias that gives the colour a hint of another colour. For example, a blue paint may have a green or a red undertone or bias. When mixing paint, consider both the masstone and the undertone. Mixing two paints with the same undertone will create a cleaner undertone. For example, blue and yellow paints with blue undertones will create a clean green when mixed. Primary colours, also available in gouache, are those colours lacking an undertone, where the colour falls midway between the other two primary colours.