• Mixing the broad distributions of green and red yields yellow. Although the resulting spectral distribution is very different from spectral yellow.

• If one mixes roughly equal amounts of monochromatic green and red, the result also looks yellow, although the spectral yellow is completely absent.

• Two colors which look alike even though they have different intensity distribution curves are called metamers.

Adding Colors

• Green + Red = Yellow

• Blue + Green = Cyan

• Blue + Red = Magenta (purple)

• Blue + Green + Red = White

Simple Additive Rules

Red

Blue Green

YellowMagenta

White

Cyan

The primary additive colors are red, green, and blue.

Complementary Colors

• Two complementary colors combine to produce white

• Cyan + Red = W (C = B + G)

• Blue + Yellow = W (Y = G + R)

• Magenta + Green = W (M = B + R)

• Some spectral colors have complementary spectral colors. For instance, the complementary of the orange (600 nm) is bluish cyan (488 nm).

• Not all spectral colors have complementary spectral colors. For instance, the complement of green (530 nm) has to be a double humped distribution of red and blue (magenta).

Chromaticity Diagram

• Question: if given spectral blue, green and red, can any color (hue, saturation, brightness) be matched by mixing them?

• Almost! You can’t match spectral cyan.

• If you choose as your three colors: red (650nm), green (530nm), and blue (460nm) the relative amount of mix to get the spectral color is shown in Fig. 9.10

• A chromaticity diagram, a horseshoe shaped diagram, displays how any two colors can combine to form other colors.

• The information in a chromaticity diagram is similar to, but more quantitative than, that in the layer of branches at the top of the Munsell color tree. Note that white, not gray or black, lies at the center of the chromaticity diagram.

• Spectral colors (saturated, monochromatic) lie on the boundary of the horseshoe.

• All colors inside the horseshoe are unsaturated. The degree of saturation increases from 0% at white to 100% at the horseshoe boundary.

• The line of purples (or magentas) connects the two ends of the horseshoe. These are not spectral colors.

• The straight-line between two of the colors passes through all the colors that one can get by mixing the two.

• The complement of any color is found by extending a straight line from that color through white to the opposite side of the horseshoe.

More about the Chromaticity Diagram

1931 CIE Chromaticity Diagram

Chromaticityx,y (z=1-x-y)

Why do surfaces appear to have color?

• Most light sources contain all possible wavelengths in the visible light range (the sun, light-bulbs,…)

• We see colored surfaces because they absorb the light of certain wavelengths very strongly. The reflected light then has a colors complementary to the color spectrum of the absorption.– A red surface absorbs cyan very strongly.

Subtractive Mixing

• When you mix red and green paint, you don’t get yellow. You get black! This is because the red paint absorbs cyan, and green absorbs magenta. When mixed together, they absorb all wavelengths.

• Therefore the three primary colors for subtractive mixing (in pigments or filters) are not red, blue and green, they are cyan, yellow, and magenta. Let’s see why.

Ideal Subtractive Filters

100%

0%

500 nm 700 nm600 nm400 nm

100%

0%

500 nm 700 nm600 nm400 nm

100%

0%

500 nm 700 nm600 nm400 nm

Yellow filter

Magenta filter

Cyan filter

Pass blue & green Absorb red

Absorb green

Absorb bluePass green & red

Pass blue Pass red

Combining Subtractive Filters

WhiteLight

WhiteLight

WhiteLight

Cyan Light

Cyan Light

Yellow Light

Cyanfilter

Magentafilter

Cyanfilter

Yellowfilter

Yellowfilter

Magentafilter

Blue Light

Green Light

Red Light

(B+G+R)

(B+G+R)

(B+G+R)

(B+G)

(B+G)

(G+R)

Simple Subtractive Rules

• Mixing Cyan + Magenta pigments, one gets Blue

• Mixing Cyan + Yellow pigments, one gets Green

• Mixing Magenta + Yellow pigments, one gets Red

• Mixing Cyan + Yellow + Magenta pigments, one gets Black

Magenta

Cyan Yellow

RedBlue

Black

Green

Dependence of subtractive color on the light source

• The color of the light reflected from an object also depends on the light source. – Under the “golden white” sodium lamps on

highways, some objects lose their color because there is little green and red light in it.

• To find the color of an object under a non-white light source, we need to know the intensity distribution of the light source and the absorption spectrum of the surface.

• Even if two illuminating lights look the same, an object may still appear different colors in them. – Under a white light consisting of two narrow

bands of cyan and red, the yellow object can only appears as red or black.

– However, the same yellow object appears yellow under sun light.

Partitive Mixing

• Placing small separate sources close to each other. Your eyes do not see the separate sources, but the mixed color. – Color TV: one picture tube but three

electron guns. Electrons from different guns are directed to different points on the screen. The screen consists of dots of three different phosphors, each of which will produce one of the additive colors.

• Pointillist painters: put small dabs of different color paint near each other, and look at the picture from a distance– Mosaic– Stained glass– Trees in the fall– Textiles

A Sunday on La Grande Jatteby Georges Seurat - 1884

Positive afterimage

• Put the colors “near each other” in time. The colors change so rapidly that the positive afterimage of one mixes additively in your eye with the image of the next.– Rapidly rotating wheel with different color

segments (Color wheel)

Your eyes are exposed to pictures of different colors!