Undercoat n. : A coat worn under another; a light coat, as distinguished from an overcoat, or a greatcoat.

Undercoat n. : A growth of short hair or fur partially concealed by a longer growth; as, a dog's undercoat.

Example:
What dog breeds have a gray Undercoat under their top coat?

Taxonomy
hairs, Down hair, free encyclopedia, Undercoat, layer, search, Unsourced, ground, trapping, insulating, collectively, Mammal, Personal tools, Interaction, Toolbox, Languages, non-profit organization, dog shows.

Defination from the web:
... of animal fancy, the down hair is collectively called the undercoat and may be important in judging the animal, particularly at conformation dog shows and cat shows. ...