What article is used before yellow?

The article used before "yellow" depends on whether you are using it as a singular countable noun or a color adjective.

Singular countable noun:

* A yellow car (referring to one specific car that is yellow)

Color adjective:

* The yellow house (referring to a specific house that is yellow)

* No article needed when yellow is used to describe something: The sky is yellow.

So, the answer is "a" or "the", depending on the context.