Mellis studied at Edinburgh College of Art from 1929-33, and won a travelling scholarship to Paris. In 1938 she met the critic and painter Adrian Stokes, whom she married. In 1939 they moved to Cornwall, both becoming central figures in the St Ives group of artists. She later married the artist Frances Davidson and they spent time in France in the later 1940s. In 1950 Mellis moved to Suffolk where she still lives
and works. Marsh White is one of a group of paintings
geometric shapes which were concerned with colour, texture
and space. Mellis commented that the white area at the bottom left was 'like a snow field' and that the orange area at the top was 'like a sunset on the marsh'.