What is Color?
There are two types of hadrons: baryons and mesons. Every baryon is made up of three quarks and every meson is made of a quark and an antiquark. For example, the proton is composed of two up quarks and a down quark (uud). All quarks have the same quantum numbers for such properties as spin, size, parity, etc. Therefore, according to the Pauli Exclusion Principle which states that no two identical objects can occupy the same place, it is impossible for one particle to contain two of the same kind of quark. Yet the proton contains two up quarks. Because of this contradiction, it was proposed that quarks must have another property with six manifestations. This new property was labeled color (which should not be confused with the common understanding of color). The six manifestations are termed red, blue, green, antired, antiblue, and antigreen. The anti-colors belong, appropriately, to the antiquarks. To obey the Exclusion Principle, all three quarks in a baryon are of different colors and a meson must contain a colored quark and a quark of the corresponding anti-color. A good place to learn more about baryons, mesons, quarks, etc. is the the Chart of the Standard Model of Fundamental Particles and Interactions .
Experimental Evidence and Color Combinations.
While quarks have color, the particles that they make up are colorless. The red, blue, and green quarks present in every particle come together to make a colorless particle, much as red, blue, and green light form white light when combined. A meson, on the other hand is composed of a red quark and an antired antiquark, who's colors cancel each other out.
There is, in fact, some experimental evidence for the existence of color. Both the rate at which [pi]0 decays into two photons and the probability that electrons and positrons will create hadrons when they collide indicate that there are three times as many quarks as would be expected if color did not exist. Theoretical predictions are sensitive to the number of quark species (colors), so if there are in fact three colors of quarks, then the prediction agrees with the determined number, and all is well.
How does the strong force relate to color?
One of the four fundamental forces is the strong force, which holds quarks together. This strong interaction is completely based on color, just as electromagnetic interactions are bsed on electric charge. Quantum chromodynamics (QCD) is the current local gauge theory used to describe how quarks have strong interactions. A gauge theory is a symmetry relating to internal properties of particles, and in the case of quarks, that symmetry is color. Color, like charge, is a conserved quantity which cannot be created or destroyed. The main difference between QCD and quantum electrodynamics is that charge is replaced by color. Still, it is possible to use many of the concepts of quantum eletrodynamics in quantum chromodynamics.
Quantum chromodynamics solves many problems encountered in this type of particle physics. Meson and baryon spectra, quark statistics (as described in the above section), scale invariance of interactions at short distance, and quark confinement at large distances all make sense when the ideas of QCD are used. A page all about Quantum ChromoDynmaics can be found here.
How does the color (strong) force work?
Colored quarks attract one another by exchanging gluons, of which there are eight types. Gluons are massless, have spin 1, travel at the speed of light, and carry both a color and a different anticolor. The number of gluons is determined by the number of color-anticolor combinations. If gluons did not carry color, they would not obey the gauge field theories or observe asymptotic freedom. When a quark emits or absorbs a gluon, it changes color, a process that happens constantly. Therefore a proton (uud), which must have all three colors, can have several different arrangements. Examples are urugdb, uburdg, ugubdr.
There are imaginary field lines between quarks much as there are in quantum electrodynamics. These field lines are composed of many gluons. The gluons making up the imaginary field lines all possess color charge, and therefore are capable of attracting each other. Photons, in the quantum electrodynamics model, possess no charge and are imcapable of attracting one another. Whereas the force between electrons decreases as distance increases, as the distance between two quarks increases, the color force binding them together also increases. A good example illustrating this concept is that of streching a rubber band. The more it is streched, the more effort it takes to strech it further. Since the color chrage is carried by gluons which can attract each other, the color charge is spread out rather than localized. It is shared among gluons. Due to this, the effective color charge will apear larger at long distances.
Confinement of quarks.
Due to the fact that the strong force increases with distance, confinement of quarks exists. There are no free quarks; no quarks have the ability to break away from their hadron. However, within a very small area, quarks have the freedom to move anywhere. This property is called asymptotic freedom.
All hadrons are composed of groups of quarks. They are formed when a quark and an antiquark move away from each other and new quark-antiquark pairs are formed due to the increase in energy required to separate the original quark and antiquark. This concept can be thought of as a magnet with a north and south pole that is broken into pieces. The pieces each still have a north and south pole. So it is with quarks when the color force is broken. Color is still conserved and each particle is colorless.
Another product of color is residual strong interaction, which accounts for protons sticking together in the nucleus of an atom even though they all have the same electric charge. The quarks of one proton become glued to the quarks of another proton and the attraction between the quarks is strong enough to overcome the repulsion between the charged protons.
Color, then, is the property of quarks that allows two otherwise identical quarks to exist in the same particle simultaneously. Color also is the source of the strong, or color, force which binds quarks together. Quarks exchange gluons, which are also colored, in order to stick together and in so doing, change color themselves. Quantum Chromodynamics is the theory explaining this strong force and it is very similar to the more familiar Quantum Electrodynamics. The major difference being that color replaces electric charge.
High Energy Physics and Elementary Particles Discoveries is an interesting page about the history of physics.
References
Watkins, Peter. Story of the W and Z. Cambridge University Press, Cambridge, London: 1986.
Gottfried, Kurt and Victor Weisskopf. Concepts of Particle Physics, Volume 2. Oxford University Press, New York: 1986.
Lederman, Leon. The God Particle. Houghton Mifflin Company, Boston: 1993.
Beiser, Arthur. Concepts of Modern Physics. McGraw-Hill Book Company, New York: 1987.
Ohanian, Hans. Modern Physics. Prentice-Hall, Inc., Englewood Cliffs: 1987.
Parker, Sybil. Nuclear and Particle Physics Source Book. McGraw-Hill Book Company, New York: 1988.
Trefil, James. From Atoms to Quarks. Charles Scribner's Sons, New York: 1980.