Mix Winsor Blue with a little Viridian Green then paint a circular area of about 2 inches diameter. Surround this with a a large area of Cadmium Yellow and the inner circle will appear quite blue. Now surround the Cadmium Yellow with deep Ultramarine Blue and the circle will appear green.
Repeat for an inner circle of Cadmium Red, a Cadmium Yellow background will appear red, but on an Ultramarine Blue will appear orange. Red on Grey will make the grey appear to be cold, but blue on grey will make it appear to be warm. Discover other contrasts.