Yellow is the color you'll see in a rainbow, right between orange and green. Yellow is the color of daffodils, lemons, and the traffic light that is supposed to make cars slow down.
The word yellow comes from a Proto-Indo-European root word that means "to shine." Sunflowers, egg yolks, and corn on the cob all share this bright color, and if you're asked to draw a picture of the sun, you're likely to grab a yellow crayon. The informal meaning of yellow, "cowardly," has been around since the 1850s, and may have originated in a Texas slur for Mexican soldiers, whose uniforms were yellow.