Rimsky-Korsakov’s key-color associations are preserved in the literature around him: E major as blue, sea and night sky in his operas written in it, A major as youthful and rosy, and so on. The accounts come from his own remarks and the memoir record of his circle, made in passing as working facts rather than as a promoted system.
The famous detail is the disagreement: his colors and Scriabin’s conflicted key by key, which contemporaries found notable and modern research finds predictable, since genuine synesthetic mappings are personal.
The casual, unsystematic way his colors surface in the record is itself evidence for perception over doctrine, and he is generally treated as one of music history’s more credible historical cases.