This is the 16681st most frequent Russian word.
синим
The Russian word "синим" is the instrumental case of "синий," meaning "blue" (describing color).
In this sentence, 'синим' is instrumental case used to describe the means or manner by which something occurs (the sky turned blue).
Небо окрасилось синим цветом перед закатом.
The sky turned blue before sunset.
Here, 'синим' is also instrumental case, describing the tool ('a blue pencil') used for drawing.
Он нарисовал море синим карандашом.
He drew the sea with a blue pencil.
In this example, 'синим' functions as a predicate adjective in instrumental case, matching the subject's state or description ('his coat was blue').