Just like electrons have charge, quarks have color. Of course, they are not actual colors: those are simply wavelengths of electromagnetic radiation. We call them colors because there are three of them, so it seemed a good way to label them. So “quark color” is a funny way of talking about quark charge.
There are two types of electrical charge: plus and minus. This seems natural, but only because you’re used to it. Why aren’t there three types? Or forty? This is the case for the strong interaction: there are three types of “charge” that we call “blue”, “red” and “green”. In fact, there’s also the “anti” version of each, so we have six types: blue and anti-blue, red and anti-red, green and anti-green.
The difference between the strong interaction and the electric force is that the strong interaction is much stronger (hence the name). Imagine for a moment that the electric force was really, really strong: would you ever see a charged object? Probably not: if the force was so large, it would be almost impossible to separate negative and positive charges, so you would only see neutral objects. Anything that got a charge would instantly be attracted to something with the opposite charge and get neutralized.
This is what happens with the strong interaction: it is so strong that we never see a combination of quarks that is not “neutral.” In this case, “neutral” means that the color combination has to add up to white. In fact, it is impossible to see a quark on its own: this phenomenon is called confinement.
Hadrons, baryons and mesons
Now, there are two ways of making “white” particles. The first one is by taking three different quarks with a different color each: red, blue and green make white, just like with actual, real-life colors. The other way is to take two quarks with opposite colors: blue and anti-blue, red and anti-red, etc. Just like opposite charges cancel out, so do opposite colors. All particles made of quarks are called “hadrons.”
The proton is an example of a white particle with three quarks. We call these particles “baryons.” A proton has two “up” quarks and a “down” quark. Each quark has a different colour, like in the picture to the right. The neutron is made of two “down” quarks and an “up” quark and is also “white” like the proton. In fact, as far as the strong force is concerned, the proton and neutron are pretty much the same particle.
The other type of white particle is called a “meson”. Mesons are made of a quark and an anti-quark, so that the anti-quark has an anti-color. For example, the π+ is made of an up and an anti-down quark. If the up is blue, then the anti-down will be anti-bue and so on. There are many different mesons which typically do not last much.