This is the 16679th most frequent Russian word.
синего
"Синего" is the genitive or accusative singular form of "синий," meaning "blue" in English.
Here, "синего" is used as a genitive form of the adjective "синий" (blue) to describe the color of the scarf.
Шарфик синего цвета выглядел прекрасно.
The scarf of blue color looked wonderful.
In this sentence, "синего" is in its genitive case to attribute the color "blue" to the observed light.
Мы увидели отблеск синего света в темноте.
We saw the glow of blue light in the darkness.
The usage of "синего" here, in the genitive form, describes the shade of the automobile, emphasizing a characteristic property of the object.