Definitions of yellow:

• noun:   the quality or state of the chromatic color resembling the hue of sunflowers or ripe lemons
• verb:   turn yellow

  Example: "The pages of the book began to yellow"

• adjective:   cowardly or treacherous

  Example: "The little yellow stain of treason-M.W.Straight"

• adjective:   changed to a yellowish color by age

  Example: "Yellowed parchment"

• adjective:   similar to the color of an egg yolk
• adjective:   affected by jaundice which causes yellowing of skin etc
• adjective:   easily frightened
• name:  A surname (very rare: popularity rank in the U.S.: #30865)