How many letters does the top answer for “author of theory of colours” have?

The most common answer is goethe (6 letters).

Crossword clue: author of theory of colours

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

goethe 6 letters wd-fact-v3

German poet and novelist and dramatist who lived in Weimar (1749-1832)

author of aesthetic theory 50% similar → adorno

author of japhetic theory 50% similar → marr

wrote theory of colours 50% similar → goethe