Complex sentence: He developed an interest in "colour-music," the relationship of colour harmony to musical harmony, and in 1919 held a joint exhibition with Roland Wakelin titled "Colour in Art" to expound his theories.
Split into simpler sentences: He developed an interest in "colour-music," the relationship of colour harmony to musical harmony. In 1919 he held a joint exhibition with Roland Wakelin titled "Colour in Art" to expound his theories.