In the arts of painting, graphic design and photography, colour theory is a body of practical guidance to colour mixing and the visual impact of specific colour combinations. Although colour theory principles first appear in the writings of Alberti (c.1435) and the notebooks of Leonardo da Vinci (c.1490), a tradition of ‘colour theory’ begins in the 18th century, initially arising from the  controversy around Sir Isacc Newton’s theory of colour (Opticks, 1704) and the nature of so-called primary colours. From there, it developed as an independent artistic tradition with only sporadic or superficial reference to colorimetry and vision science. When it comes to the mixing of paint, Newton’s colour wheel is often used to describe complementary colours, which are those which cancel each other’s hue to produce an achromatic (white, grey or black) mixture. Newton offered as a conjecture that colours exactly opposite one another on the hue circle cancel out each other’s hue, but it became apparent 19th century that this is a basic fact of colour vision.