You are looking at the magnetic spectrum.
If the wavelength is between or equal to 400 and 500 nm, you return the color "BLUE".
If the wavelength is greater than 500 nm and less than or equal to 600 nm, you return the color "GREEN".
If the wavelength is greater than 600 nm and less than or equal to 700 nm, you return the color "RED". If it is not between 400 nm and 700 nm, then return "INVISIBLE".