The Higgs Boson: Searching for the God Particle by Scientific American Editors Page A

The binding of a quark and an antiquark would then be explained on the principle that opposite charges attract, just as they do in the hydrogen atom. The structure of the baryons, however, is a deeper enigma.
To explain how three quarks can form a bound state one must assume that three like charges attract.
    The theory that has evolved to explain the strong force prescribes exactly these interactions. The analogue of electric charge is a property called color (although it can have nothing to do with the colors of the visible spectrum). The term color was chosen because the rules for forming hadrons can be expressed succinctly by requiring all allowed combinations of quarks to be "white," or colorless. The quarks are assigned the primary colors red, green and blue;
the antiq uarks have the complementary
"anticolors" cyan, magenta and yellow.
Each of the quark flavors comes in all three colors, so that the introduction of the color charge triples the number of distinct quarks.
    From the available quark pigments there are two ways to create white : by mixing all three primary colors or by mixing one primary color with its complementary anticolor. The baryons are made according to the first scheme: the three quarks in a baryon are required to have different colors, so that the three primary hues are necessarily represented.
In a meson a color is always accompanied by its complementary anticolor.
    The theory devised to account for these baffling interactions is modeled directly on quantum electrodynamics and is called quant um chromodynamics.
It is a non-Abelian gauge theory.
The gauge symmetry is an invariance with respect to local transformations of quark color.
    It is easy to imagine a global color symmetry. The quark colors, like the isotopic-spin states of hadrons, might be indicated by the orientation of an arrow in some imaginary internal space.
Successive rotations of a third of a turn would change a quark from red to green to blue and back to red again. In a baryon,
then, there would be three arrows,
with one arrow set to each of the three colors. A global symmetry transformation,
by definition, must affect all three arrows in the same way and at the same time. For example, all three arrows might rotate clockwise a third of a turn.
As a result of such a transformation all three quarks would change color, but all observable properties of the hadron would remain as before. In particular there would still be one quark of each color, and so the baryon would remain colorless.
    Quantum chromodynamics requires that this invariance be retained even when the symmetry transformation is a local one. In the absence of forces or interactions the invariance is obviously lost. Then a local transformation can change the color of one quark but leave the other quarks unaltered, which would give the hadron a net color. As in other gauge theories, the way to restore the invariance with respect to local symmetry operations is to introduce new fields.
In quantum chromodynamics the fields needed are analogous to the electromagnetic field but are much more complicated;
they have eight times as many components as the electromagnetic field has. It is these fields that give rise to the strong force.
    The quanta of the color fields are called gluons (because they glue the quarks together). There are eight of them, and they are all massless and have a spin angular momentum of one unit.
In other words, they are massless vector bosons like the photon. Also like the photon the gluons are electrically neutral,
but they are not color-neutral. Each gluon carries one color and one anticolor.
There are nine possible combinations of a color and an anticolor, but one of them is equivalent to white and is excluded, leaving eight distinct gluon fields.
    The gluons preserve local color symmetry in the following way. A quark is free to change its color, and it can do so independently of all other quarks, but every color transformation must be accompanied by the