Maybe you've created your own custom color from paints lying around the basement, only to find that you can't figure out how to replicate it. Or perhaps you moved into a house with a paint color scheme that you love, but it needs some touching up.
You head over to your local paint store and come home armed with a pocketful of paint chips, thinking surely one of the colors will match, but no luck. Who knew there were so many different shades of green?
As we learned from our big box of Crayolas, purple isn't just purple and red isn't just red. You can choose from cornflower blue, midnight blue, blue-gray, blue-green and the list goes on. Colors are actually waves of light, and each wavelength has a value that makes it different from the rest. A color can be described by how pure or desaturated it is, as well as its value or lightness.
So describing something as green or even blue-green isn't going to be specific enough if you're trying to find the exact color.