Can the word yellow be a noun?

Yes, the word "yellow" can be a noun in certain contexts.

Here are some examples:

* Color: "The artist used a vibrant yellow in the painting." (Here, "yellow" refers to the color itself.)

* Substance: "The fabric was dyed a bright yellow." (Here, "yellow" refers to the substance used for dyeing.)

* Group: "The yellow team won the championship." (Here, "yellow" refers to a team or group identified by that color.)

However, "yellow" is most commonly used as an adjective, describing something that is of that color.