Color charge is the property of quarks and gluons responsible for their participation in the strong interaction. Its theory, quantum chromodynamics (QCD), forms part of the Standard Model of particle physics. Quark color states are conventionally called red, green, and blue, but these names have no connection with visible color. They label an internal quantum property rather than a particle’s appearance. Gluons carry color themselves and mediate interactions among color-charged particles. (pdg.lbl.gov)
Colors and anticolors
Each of the six quark flavors has three color states. Color and flavor are separate classifications: an up quark, for example, can occupy any of the three color states without becoming a different flavor. Antiquarks have corresponding anticolor states, conventionally called antired, antigreen, and antiblue. The color labels describe three components of a quantum state, not three substances inside a quark. (home.web.cern.ch)
Color is often compared with electric charge, which determines electromagnetic interactions. The analogy is useful but incomplete. Electric charge is described by a single charge operator, whereas QCD involves eight generators acting on color states. Moreover, the photon, which mediates electromagnetic interactions, is electrically neutral, while gluons carry the charge associated with their own interaction. This distinction permits direct gluon self-interactions. (arxiv.org)
Mathematical description
QCD is a gauge theory with the symmetry group , usually written to distinguish color from other uses of the same mathematical group. In the language of group theory, quarks transform in its three-dimensional fundamental representation, antiquarks in the conjugate representation, and gluons in its eight-dimensional adjoint representation. (pdg.lbl.gov)
A quark’s color state belongs to a complex three-dimensional vector space. Red, green, and blue name a chosen basis. A state can be a quantum superposition of these basis states; assigning a particular color therefore depends on the basis used. Local gauge transformations change the description without changing physical predictions. Color is consequently not a directly observable paint-like label on an isolated particle. (damtp.cam.ac.uk)
The eight generators are represented by matrices. Their action describes how a gluon interaction changes a quark’s color state. Because these generators do not generally commute, QCD is called a non-Abelian gauge theory. Its quantum field theory includes both three-gluon and four-gluon interaction vertices. (pdg.lbl.gov)
Why there are eight gluons
The familiar statement that gluons carry a color and an anticolor is a useful shorthand. Three colors combined with three anticolors span nine states, but these decompose as
The eight-dimensional part corresponds to the gluon color states. The remaining component is a color singlet and is not an additional QCD gluon. Thus QCD has eight gluon fields, not nine. (damtp.cam.ac.uk)
Six basis states can be represented by unequal color–anticolor pairs. The other two are independent traceless combinations of matching pairs. A gluon described using matching colors is therefore not necessarily color-neutral: a true singlet must remain unchanged under every color transformation, not merely have matching labels in one basis. (damtp.cam.ac.uk)
Color neutrality and hadrons
Observable hadrons are color singlets. A conventional meson contains a quark and an antiquark whose color wave function is proportional to
A conventional baryon, such as a proton or neutron, has three valence quarks with an antisymmetric color wave function involving all three colors. These are quantum combinations, not literal mixtures of colored objects. (damtp.cam.ac.uk)
Color neutrality does not imply electrical neutrality: protons are color-neutral but electrically charged. Nor does it mean that a hadron contains no gluons. Its internal structure includes gluons and additional quark–antiquark contributions, while the complete state remains a color singlet. (energy.gov)
Confinement and interaction strength
Color confinement describes the absence of isolated quarks and gluons among freely observed particles. In the flux-tube description, separating color sources concentrates the field between them, increasing its stored energy. With dynamical quarks, sufficient energy can produce new quark–antiquark pairs, allowing the system to form additional color-neutral hadrons rather than release a lone quark. (damtp.cam.ac.uk)
At large momentum-transfer scales, the strong coupling becomes smaller, a property called asymptotic freedom. This makes perturbation theory useful for many high-energy processes. At hadronic scales, the coupling is stronger, and nonperturbative approaches such as lattice QCD become important. The scale dependence concerns the interaction strength; it does not alter the number of quark colors or gluon states. (arxiv.org)
Historical development and experimental evidence
An additional three-valued quark property emerged during the 1960s to address problems involving quark statistics and the Pauli exclusion principle. Proposals associated with Oscar Greenberg and with Moo-Young Han and Yoichiro Nambu helped establish the additional degree of freedom later incorporated into QCD as color. The discovery of asymptotic freedom in 1973 supported the development of QCD as the theory of strong interactions. (nobelprize.org)
Although individual color labels are not measured directly, color has experimentally testable consequences. Rates for electron–positron collisions producing hadrons contain a multiplicity factor associated with the three quark colors. Gluon radiation also produces particle jets through hadronization. Three-jet events observed at PETRA in 1979 provided evidence for gluon emission, while jet distributions and scattering rates test the color structure of QCD quantitatively. (arxiv.org)