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Name: Leslie
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 1999-2001

Can you please explain (in middle school terms) why in TV red, blue, and green light are considered primary colors and when mixed create white light while in art, the primary colors are red, blue, and yellow and when mixed don't come close to white. If fact if mixing red, blue, and green paint I wouldn't get white either. Why is that?

Pigments absorb light, reflecting only the color that you see. For example, when you mix blue and yellow pigments together, you are mixing together compounds that absorb blue light (leaving yellow and red, which basically looks like yellow) and that absorb red (leaving blue). The combination absorbs both blue and red, (that's both ends of the spectrum), leaving only green (middle of the spectrum) to be reflected. When you combine blue and yellow lights, you see both the blue and the yellow, which is composed of different wavelengths than the color reflected from the mixed pigments.

Color perception is kind of a funny thing; what actually happens is that cone cells in our retinas are sensitive to different ranges of wavelengths: basically, red (long wavelengths), green (middle), and blue (short wavelengths). Their sensitive regions overlap a little, and your brain processes the inputs from the different sensors to reconstruct about what the color must be. So, if you see a monochromatic light that's in the region two types of cones respond to, you see it as one color. Yellow light, for instance, excites both red- and green-sensitive cones. Mixing together monochromatic red and monochromatic green light will ALSO excite both the red and green cones, and your brain will see yellow. As far as it knows, it's the same color as the monochromatic yellow, but a spectrometer could easily tell the difference.

Richard Barrans, Ph.D.
Assistant Director
PG Research Foundation, Darien, IL

The main difference is that TV's, Computer screens, that have Red, Green, Blue, as their primary colors give off light. In a dark room, you can still see if a TV or computer is on because the screen gives off light, like a light bulb. In fact, the screen is made up of thousands of tiny red, green, and blue lights that are so close together you can hardly see them. By turning on a Red, Green, and Blue lights that are really close, the TV can mix the lights and create all kinds of different colors. To figure out what colors to make, TVs start with black, and begin turning on lights, or adding colors to get what they want.

The main point is that the Red, Green, and Blue colors a TV uses, produce light and make colors by adding colors to the light we see.

With art, it is just the opposite. Paper and paint reflect light. White paper reflects all the light that shines on it pretty much the same. But colored paints do not do this, they reflect some colors better than others, and absorb the rest. Black paint reflects hardly any light and absorbs all colors pretty much the same. Red paint absorbs all colors but red light, and reflects just red light. Yellow paint absorbs everything but yellow, and so on. So when you mix two colors, you are deciding which colors to remove from the light we see. Paint basically subtract light. Red paint subtracts (or doesn't reflect Green and Blue). Yellow subtracts Blue, and, Blue subtracts Green and Red. Computer printers use this kind of system where they use Cyan, Yellow, and Magenta, which are the exact opposites of the Red, Green, and Blue that TVs use. Cyan subtracts Red, Yellow subtracts Blue, and Magenta subtracts Green. So, in art, you start with a with piece of paper or canvas (this is why colors sometimes look different when you use colored paper) and start removing or subtracting light to get the color you want. This is why if you mix a lot of different colors together it keeps getting darker.

So, the main point with paints is that paints make colors by only reflecting some light and removing (or subtracting) the rest.

Thanks, I hope this helps.

--Eric Tolman

I do not have a huge amount of time to answer this in detail. If my schedule frees up a bit, I will give a longer answer. You have hit on a common misconception. In color addition, the primary colors are red, green, and blue. This is how a color monitor works. If you are looking at color subtraction, as in the printing process, the primary colors are yellow, magenta, and cyan. I would suggest you read the following sources:

Paul Hewitt, Conceptual Physics, 8th edition, Addison Wesley, 1998, chapter 26

There was an article in The Physics Teacher in the April 1999 that addressed this also, but I do not have the exact cite. This is a monthly magazine put out by the American Association of Physics Teachers, and you may be able to search for it on the web.

---Nathan A. Unterman

I wrote an article entitled "Confusing Color Concepts Clarified" about this that was published in The Physics Teacher magazine in April 1999, pp. 204-206.

In a simple model, the color that you see depends on the "color" of light that enters your eye. When red light and green light and blue light enter your eye, your brain interprets the resultant color as white.

In art, the primary colors are stated to be red, yellow, and blue. However, this is incorrect. The correct primary colors are cyan, magenta, and yellow, just as in color printing. If you paint, you will find that using cyan, magenta, and yellow paints can produce many more colors than using red, yellow, and blue paints. Try it and see. The paints absorb certain colors: cyan paint absorbs red light, magenta paint absorbs green light, and yellow paint absorbs blue light. Paints are always painted on a white background which reflects all colors of light. So if you have cyan paint on top of magenta paint on top of yellow paint, here is what happens: The white light (composed of red, green and blue light) enters the cyan paint. The cyan paint absorbs the red light leaving green and blue light. The green and blue light then enters the magenta paint which absorbs the green light leaving blue light. The blue light then enters the yellow paint which absorbs the blue light. The remaining light then reflects off the white paper, passes through all the layers of paint again, and then enters your eye. But since the paints absorbed all of the incoming light, there is no light left to enter your eye. So mixing cyan, magenta, and yellow paints results in a black color.

Dr. Lawrence D. Woolf
General Atomics
3550 General Atomics Court; P. O. Box 85608
Mail Stop 15/242
San Diego CA 92121-1194
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