Remember we learned before about measuring waves using 
wavelength.

(from: wikipedia 
sine)
And we learned that how often something happens in a given amount of time is called a 
frequency.

(from: wikipedia - 
frequency)
Well did you know that the colors of the rainbow are all different because of their 
wavelength and 
frequency?
For example, the color red has a very long 
wavelength, and a very low 
frequency.
But the color violet has a very short 
wavelength, and a very high 
frequency.
That's why the colors of the rainbow are in the order they are,
and why we put them in the order of 
ROY G BIV.
Red is the longest 
wavelength and lowest 
frequency.
Violet is the shortest 
wavelength and highest 
frequency.
Green is right in the middle.
Sometimes it's easier to understand by looking,
so here's a good picture that helps show how the different colors have different 
wavelengths and frequencies: