Rainbows are an arc of color that appears in the sky after certain weather conditions. Water in the air acts as a prism, splitting sunlight into its component colors and reflecting those colors back to the viewer. Humans perceive seven distinct colors in every rainbow, although sometimes it is hard to see all of the colors at once when viewing an actual rainbow. The colors always appear in the same order.
Red is the first color of a rainbow, the color that forms its outer arch and has the longest wavelength. It is one of the most prominent colors to appear to the human eye when looking at a rainbow. In most situations, you will always be able to see the red in a rainbow, even if other colors are not apparent.
Orange is always the second color of the rainbow, appearing next to red. It is sometimes difficult to discern the orange in a rainbow because it blends in so much with the red. Many times it looks like the red in the rainbow simply fades off, when really it is fading into orange.
Yellow is the next color in the rainbow after orange; it is a very prominent color in most rainbows. It tends to stand out and is usually visible, but yellow is sometimes harder to see in light-colored rainbows or in rainbows that appear next to a particularly bright sunny sky, as the yellow color can fade away.
Green can be found in every rainbow next to yellow. It is sometimes difficult to see if the rainbow is against a bright blue sky, but it is a more prominent color than others. It tends to stand out because it is the first "cool" color of the spectrum; it contrasts with the warm colors that sit atop it in the rainbow.
Blue is the fifth color of the rainbow and one that can easily go unnoticed; many times the blue in a rainbow can blend in with the blue in the sky. Blue is more easily seen in rainbows that appear when they sky is more gray in color.
Indigo is the second to last color of the rainbow and is an intermediate wavelength between those of the two colors on either side of it, blue and violet. Indigo is one of the most easily forgotten colors of the rainbow because it is often unseen by the human eye when looking at a natural rainbow.
Violet is the final color of the rainbow, making up the inner outline of the rainbow's arc. Its wavelength is the shortest of the rainbow colors. At times it can blend in with the blue and indigo of a rainbow, but it is often visible to the naked eye and will stick out from the rest of the colors enough to be noticed. Many times the bright violet color of the rainbow will stand out against white clouds or grayer skies.
The most common method of memorizing the colors of the rainbow is to use a mnemonic device, a mental shortcut. In the case of the colors of the rainbow, the mnemonic device is the fictional name "Roy G. Biv." Each letter in this name stands for the color of the rainbow in the order it appears.