This is the 9956th most frequent Japanese word.
赤く
"赤く" means "red" or "reddishly" (adverbial form of "赤い," meaning "red").
赤く is used as an adverb to describe the way the sky is dyed, emphasizing the reddish color.
空が赤く染まっています。
The sky is dyed red.
赤く functions as an adverb here, describing the change in color of his face to red due to reaction or emotion.
彼は顔が恥ずかしさで赤くなった。
He became red-faced due to embarrassment.
赤く is used as an adverb modifying the verb なった (turned), describing the color change of the apple to indicate ripeness.