Amy Beach, the first American woman to succeed as a composer of large-scale art music, is one of the best documented historical synesthetes. Family records describe her, as a small child, requesting pieces by color, the blue music, the purple one, before she had the vocabulary of keys. The associations resolved into a stable key-color mapping she reported throughout her life: C major white, G major red, A major green, A-flat major blue, with several minor keys registering as black.
Scholarship on Beach connects the mapping to her compositional practice. Works reaching for light, nature, or the spiritual lean on the keys whose colors matched the intent, which makes her catalog an unusually testable case of synesthesia steering craft.
Her account predates modern consistency testing, so she sits in the historical tier: period documentation from childhood onward, specific and stable across a lifetime, which is as strong as the nineteenth century gets.