Here are some examples where color filters are used every day:

• stained glass windows are a traditional way to decorate starting with natural light
• car tail lights, red and green stop lights, and exit signs are usually colored filters over a white light bulb
• slide projectors, overhead projectors, and movie projectors make colored displays by putting a color transparancy into a beam of white light. But this is just a complicated color filter.
• holiday tree lights are all ordinary "white" light bulbs covered with transparent colored paint
• the spot lights used to illuminate actors and dancers on a stage frequently have a colored filter in them. Why?
• Some kinds of flat-screen TV and computer monitors -- in particular, the liquid crystal displays used in lap-top computers -- make use of color filters that can be altered electrically.
• incandescent light bulbs produce light that contains too much red and not enough blue. Some light bulbs incorporate a neodymium filter that absorbs orange light, to make the light closer to sunlight-white. Of course, this means they produce less light!
• photographers sometimes use a red filter to make the sky darker, and improve the contrast with blue clouds. A red filter also will make blue haze less visible. (These tricks were more popular and useful in the days of black and white photography.)
• ultraviolet light and even blue light can damage some materials. A color filter can be used to remove these actinic rays. Sunscreen is really a kind of color filter -- to remove wavelengths we can't even see! Wine bottles are made of dark glass because they will store their contents for a long time, and light might damage them.
• cities near Mt. Palomar use low pressure sodium lights for their street lighting. This produces just one wavelength of light -- an amber yellow. The astronomers at Mt. Palomar use a color filter to remove this one wavelength from what the telescope sees; this allows them to see very dim stars without interference from the sky glow that all cities have.

A color filter isn't the best way to make colored light -- having gone to all the trouble to make a good white light, now we throw away some of it! But it is simple and flexible, and means we only have to invent the light bulb once (instead of having to invent the red light bulb, the orange light bulb, the yellow light bulb ... ). However, light-emitting diodes produce colored light directly, and much more efficiently than do incandescent lights. These are beginning to used in stop lights and automobile tail lights. The spectrum of these lights is purer: a green light-emitting diode makes just green light.
The section on color