Color mixing depends on whether you are combining light that enters your eyes or materials that absorb and reflect light. Screens use additive mixing because red, green, and blue light sources add their intensities together. Paints, inks, and filters use subtractive mixing because pigments remove certain wavelengths from white light.
This difference explains why a phone display can make bright white, while mixing many paints often makes a dark brown or black.
Understanding Physics: Color and Light Mixing
Your eyes do not measure every wavelength separately. They use three main types of cone cells, each most sensitive to a different broad region of visible light. The brain compares the signals from these cones and creates the sensation of colour.
This is why very different light spectra can look identical. A yellow lamp can produce a broad range of wavelengths, while a screen can create a similar yellow sensation with carefully chosen red and green light.
These matching combinations are called metamers. They show that colour is partly a property of the observer, not just a fixed label carried by light.
A screen pixel contains tiny controllable areas that emit light. It changes their brightness to control the cone signals in your eye. At normal viewing distance, your eye cannot resolve the separate parts of most pixels, so their light blends.
This is useful to remember when examining a display closely with a magnifier. You may see small coloured stripes or dots rather than the many colours shown in an image. The colour balance of a screen matters too.
If one type of light source is stronger than intended, neutral grey or white can appear tinted. Brightness and colour are related, but they are not the same thing. A dim light can still have a strong colour.
Materials behave differently because their colour comes from the light left after absorption. Their appearance can change under different lamps. A blue shirt may look bright in daylight but dull under a warm indoor bulb, because the indoor bulb supplies less blue light for the fabric to reflect.
This effect is called colour rendering. Shops, art studios, and photographers care about it because objects need to look reliable under the chosen lighting.
A coloured filter works by transmitting some wavelengths while blocking others. Passing light through several filters usually makes the result dimmer, since each filter removes more of the available light.
Real pigments are not perfect wavelength selectors. A paint labelled yellow reflects a range of wavelengths and absorbs another range, rather than acting like an ideal textbook pigment. Mixing paints can therefore give muddy results because unwanted absorption builds up.
Printers often add black ink for sharp dark text and deeper shadows, since combining three coloured inks does not make a clean black in practice. When learning this topic, first identify what is being mixed. Decide whether the situation involves emitted light, reflected light, or transmitted light through a filter.
Then consider the source of illumination, the material, and the response of the eye. That sequence prevents many common mistakes.
Key Facts
- Additive mixing uses light sources: red + green = yellow, green + blue = cyan, blue + red = magenta.
- In additive RGB mixing, red + green + blue = white if the three lights have balanced intensities.
- Subtractive mixing uses pigments or filters: cyan absorbs red, magenta absorbs green, and yellow absorbs blue.
- In subtractive CMY mixing, cyan + magenta = blue, magenta + yellow = red, and yellow + cyan = green.
- Ideal subtractive mixing gives cyan + magenta + yellow = black because most visible light is absorbed.
- Visible light has wavelengths from about 400 nm to 700 nm, with violet near 400 nm and red near 700 nm.
Vocabulary
- Additive color mixing
- Additive color mixing is the creation of colors by combining different colors of light.
- Subtractive color mixing
- Subtractive color mixing is the creation of colors by using pigments or filters that absorb some wavelengths and reflect or transmit others.
- Primary colors of light
- The primary colors of light are red, green, and blue because they can be combined in different intensities to make many perceived colors.
- Pigment
- A pigment is a material that gives color by absorbing certain wavelengths of light and reflecting others.
- Complementary colors
- Complementary colors are pairs that combine to make white in additive mixing or strongly absorb each other in subtractive mixing.
Common Mistakes to Avoid
- Using the same primary colors for light and paint is wrong because light uses RGB additive mixing while pigments are best described by CMY subtractive mixing.
- Saying red and green always make brown is wrong because red and green light make yellow, while red and green pigments can absorb many wavelengths and look dull or brown.
- Assuming white is a single wavelength is wrong because white light is a mixture of many wavelengths or a balanced combination of red, green, and blue light reaching the eye.
- Thinking a blue object creates blue light is wrong because most blue objects reflect blue wavelengths from the light shining on them and absorb many other wavelengths.
Practice Questions
- 1 A screen pixel has red, green, and blue channels. If the red and green channels are each set to 80 percent intensity and the blue channel is set to 0 percent, what color should the pixel appear?
- 2 A cyan filter absorbs 90 percent of red light and transmits most green and blue light. If 100 units of red light strike the filter, about how many units of red light pass through?
- 3 Explain why mixing red, green, and blue light can make white on a screen, but mixing red, green, and blue paint usually does not make white.