A mnemonic is a device some people like to use in order to help them to remeber obscure things.
A bit of additional trivia is that Isaac Newton added the Indigo color to the list of names of the colors of the rainbow because he thought there ought to be seven colors, not just six. Consider: there are seven days in a week.
Here is a more quantitative approach to the colors of the rainbow.
Wavelengths are given in nanometers (nm). One nm is 1 × 10-9 m.
|Violet||420||400 - 440|
|Blue||455||440 - 470|
|Blue-green||480||470 - 490|
|Green||525||490 - 560|
|Yellow-green||565||560 - 570|
|Yellow||580||570 - 585|
|Orange||620||585 - 630|
|Red||660||630 - 700|