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Strength To Strength
Physics is determined by forces and things that are affected by forces, which gives rise to particular behaviour. Gravity, for example, causes particles with mass to accelerate towards each other. In other words, mass is the property a particle can have, the "gravitational charge", which allows it to be affected by the force of gravity, which causes the accelerating behaviour. Different kinds of particle behaviour indicate different particle properties related to different forces.
It was the patterns in the particle data, from the time the "zoo" was being populated, that revealed the nature of the underlying forces at work. It was clear that not all particles obeyed the same forces, meaning that some had properties that others did not. This was understood even from the time when just electrons and protons were known, as a group of protons could somehow overcome their mutual electric repulsion and coexist as a group, which was not a feature of electrons. Protons, therefore, had some property that allowed them to obey some force that was even stronger than the electric force. This force came to be known, imaginatively, as the "strong force".
The electric force only has two types of electric charge, which is the property a particle must have in order for it to obey the electric force. As mentioned, these charges are labelled "negative" (as carried by electrons) and "positive" (as carried by protons and positrons). Like charges repel, and unlike charges attract. Simple. It was determined that, whereas the electric charges cancel in pairs, the strong charges must cancel either in pairs or in triplets. This calls for six types of charge rather than two, and this complexity is one of the reasons that the particle zoo was so difficult to interpret.
The electric charges were labelled as "negative" and "positive" for the very reason that these are mathematical words that provide an image of two-way cancellation to a "neutral zero". A set of mathematical words that provide an image of three-way cancellation is not possible, but this is possible with the colour words "red", "green" and "blue", as together these combine to give a "neutral white" for the strong force triplet combination. That's not to say that a particle with strong charge actually looks "red", "green" or "blue", these are just labels that work well to describe the force. The strong force pair combination is described by the further notion of each colour having an opposite with which it cancels directly, and so we have "anti-red", "anti-green" and "anti-blue", which completes the set of six strong charges mentioned above.
So why was all this needed to explain the observations? Well, interestingly, the new particles that seemed to have substructure could only be explained in terms of constituents that obeyed the strong force. These constituents, which were never seen on their own, were called "quarks", and it was reasoned that each single quark must carry a single strong charge. A number of particles could be explained as being made of two oppositely coloured quarks bound together, for example red plus anti-red, whereas others could be explained as being made of three differently coloured quarks bound together, either red plus green plus blue, or the equivalent addition of all the anti-colours.
There is one further twist to the strong force that differentiates it further from the electric force, and which is a key characteristic. The electric force allows electric charges to be free of their cancelling opposite, so we may take an electron on its own and play with it separately from a proton. But the strong force appears to demand absolute neutrality at all times, and as such it does not allow for single quarks to be pulled out from some composite particle and inspected individually. If you try to do so, the job will always require at least enough energy to create yet more quarks, which will have strong charges that cancel those of the pieces you pulled apart. It is said that "bare" colour can never be seen.
The complexity and subtlety of the strong force was the very reason that many years' worth of particle data took so long to interpret. Researchers had expected to be able to smash particles down to their very building blocks and inspect them individually, but in reality many of these building blocks were quarks and so could not be seen in isolation due to the strong force neutrality condition. We now know of six unique quarks, three of which carry positive electric charge and three of which carry negative electric charge. All can carry any of the red, green or blue strong charges, and all have a counterpart antiparticle with the opposite electric and strong charges. This all allows for a multitude of various combinations.