LEDs are not really monochromatic. Pick up a pair of diffraction grating glasses - cheap paper ones with plastic diffraction grating lens are cheap, work great, tons of fun and rather addictive and you will embarrass your wife if you wear them into a restaurant - and see for yourself. While preparing a unit on light, I looked at a whole bunch of different LEDs I had kicking around the old Laboratory for the Study of the Reanimation of Life here in Confusion and kept myself amused for longer than I would care to admit.
It's not unusual to have more than 1 color LED in a single package - especially in the water clear packages - whose light output is mixed to give a particular color of light. High quality white LEDs are made of a red, blue and green leds in a single, 2 lead package. You can even buy 4 lead devices (common ground) that let you mix the different LEDs by varying the amount of current driving each color.