Color:
* "The room was painted a mousy shade of gray."
* "Her hair was a mousy brown."
Appearance:
* "He had a mousy little mustache."
* "The kitten was so small and mousy."
Personality:
* "He was a mousy young man, always shrinking away from attention."
* "She had a mousy demeanor, quiet and unassuming."
Remember that "mousy" can sometimes have a slightly negative connotation, implying something is timid, insignificant, or lacking in character.