Mary Gartside was an English water colourist and colour theorist. She published three books between 1805 and 1808. In chronological and intellectual terms Mary Gartside can be regarded an exemplary link between Moses Harris, who published his short but important Natural System of Colours around 1766, and Johann Wolfgang von Goethe’s highly influential theory Zur Farbenlehre, first published in 1810. Gartside's colour theory was published privately under the disguise of a traditional water colouring manual. She is the first recorded woman known to have published a theory of colour.

Mary Gartside exhibited some of her own art work, paintings of flowers in watercolour, at the Royal Academy in 1781, at the Botanic Gardens in Liverpool in 1784, and at the Associated Artists in Water-Color in London in 1808. Mary Gartside died near Ludlow on 9 December 1819, aged 64.

Between 1805 and 1808 Mary Gartside published three books on painting in watercolour that reflect her interest in colour theory and its applicability. They were, in chronological order: An Essay on Light and Shade, privately published in 1805; Ornamental Groups, Descriptive of Flowers, Birds, Shells, Fruit, Insects etc,. published by William Miller in 1808; and the second, enlarged edition of her first book with the new title An Essay on a New Theory of Colour, published by Gardiner, Miller and Arch in 1808. New Theory of Colour was intended as the first of a three-volume set, but volumes 2 and 3 never appeared. A 10-page pamphlet appears to have preceded An Essay on Light and Shade, and is titled An Essay on Light and Shadow. It does not contain the hand-coloured blots included in the later editions.