This is the 9170th most frequent Russian word.
серый
Gray.
In this sentence, 'серый' functions as an adjective that describes the color of the cat, indicating it is gray.
Кот был серого цвета.
The cat was gray.
Here, 'серый' is used metaphorically to describe someone's mood, implying that it is colorless, unexciting, or gloomy.
Его настроение сегодня было серым.
His mood today was dull.
In this context, 'серый' is part of a proper noun, referring to the gray characteristic of the wolf in the title of the story.