How many letters does the top answer for “changing to a darker color” have?

The most common answer is blackening (10 letters).

Crossword clue: changing to a darker color

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

blackening 10 letters wn-def

changing to a darker color

darkening 9 letters wn-def

changing to a darker color

changing to a lighter color 50% similar → lightening, whitening