The article Synesthetic Perception: Alexander Scriabin's Color Hearing by Kenneth Peacock, in Music Perception: An Interdisciplinary Journal Vol. 2, No. 4 (Summer, 1985) details the sources. It's on JStor.
It contains a list of Scriabin's (AND Rimsky-Korsakov's) synesthetic perceptions of all the keys. They're shown as major keys. He says Scriabin objected to the terms 'major and minor' in tonality, but that shades and nuances of colour were associated with modulation and that composite colours were the result of several tonalities being combined.
It also contains C.C. von Gleich's description of the role of the Tastiera per luce part (the top stave) in Prometheus and his convincing analysis of precisely how the colours relate to the harmonies.
Leonid Sabaneyev (or 'Sabaneev'), an authority on Scriabin, had a printed copy of Prometheus, which Scriabin extensively annotated with colour markings in 1913. It was acquired by the Paris Bibliothèque Nationale in 1978.
I think this search should have found it there, but perhaps I did something wrong (Scriabine instead of Skrâbin?!). Or maybe it hasn't been digitized yet.