Non-elementary particles (mainly hadrons)
The elementary particles may be combined into bound states called composite particles. These may be complex things such as atoms, but in a particle physics context, the expression mostly refers to bound states of quarks and gluons. These are collectively called hadrons, as in Large Hadron Collider. Hadrons consist of a certain number of quarks and antiquarks called valence quarks, and in addition a lot of (anti)quarks and gluons that spontaneously show up and disappear again, a kind of quantum fluctuation. These extra quarks are called sea quarks.
There are two well-known types of hadrons: baryons and mesons. Baryons consist of three valence quarks, antibaryons correspondingly of three antiquarks. Mesons consist of a quark and an antiquark, as do antimesons. Why is it like this? Because quarks carry a so-called colour charge, and only certain combinations of colour charges can exist in nature. These allowed combinations are said to be colourless (or white). Coloured particles can only appear on their own in intermediate states of a process, not in am observable final state. This is called confinement of colour. There are three different 'colours' and three 'anticolours.' Quarks carry a colour, antiquarks an anticolour, and gluons carry one of each. The colours are sometimes labeled red, green, and blue, but you could really call them anything. Shortening the colour names to their first letters and marking anticolours with an bar over the letter, the meson and baryon colour configurations can be denoted like this:
The first letter in each term corresponds to the first quark in the hadron, the second to the second quark (antiquark in the case of mesons), etc. The fact that there are several terms means that we are dealing with a quantum mechanical superposition of states. To get the colour configuration of an antimeson or antibaryon, replace all colours by the corresponding anticolours and vice versa.
Why does nature have this property of only allowing colourless states? Nobody knows.