Some musicians and composers have a form of synesthesia that allows them to "see" music as colors or shapes. This is called chromethesia. Mozart is said to have had this form of synesthesia. He said that the key of D major had a warm "orangey" sound to it, while B-flat minor was blackish. A major was a rainbow of colors to him. This may explain why he wrote some of his music using different colors for different music notes, and why much of his music is in major keys.