A tertiary colour is made by mixing equal amounts of a primary colour and a secondary colour together. There are six tertiary colours. On the colour wheel, they sit between the primary and secondary colour they are mixed from.
In Woman with Parasol (1893), Paul Signac creates a vibrant image based on tertiary colours.
Instead of mixing different colours of paint Signac applied small dabs of red-orange, blue-green, red-purple (and yellow) to his canvas. From a distance our eyes mix these to see different hues, tints and tones.
Using pure contrasting tertiary colours results in an intense, bright image.