Scriabin's decision to orchestrate his fifth symphony "Prometheus" with a "counterpoint of light" resulted from his perception of sound as literal color. This phenomenon is known as synesthesia, and by the early decades of this century, well over 100 specialized case studies had appeared in the experimental literature. The present article is in two parts. The first is a general discussion of the vast literature on synesthesia. With this perspective, Scriabin's color hearing can be understood to have resulted from a typical synesthetic pairing of diverse sensory stimuli. In part two, the composer's personal perception is examined, and an analysis of the "Tastiera per luce" in the orchestral score is presented. This part for colored light serves a dual function by indicating particular colors to be projected during performance as well as all transposition levels of the six-note pitch collection employed exclusively in the composition.