Colour was not detected before this because all quarks obey confinement. This means that quarks only combine in ways that are colour neutral overall. This is slightly analogous to charged particles. A negative particle and a positive one attract each other, and will combine to form a neutral one. The essential difference is that we may observe positive and negative charges, but we never see 'coloured' systems.
Some particles are colour neutral but have only two quarks.
This is achieved by saying that each colour has a corresponding anti-colour, which cancels it out. Thus a particle could be composed of a red and an anti-red quark, and be colour neutral. Particles Bloomsbury Guide to Human Thought 2. James, Jessica JJ Bloomsbury Guide to Human Thought 3. Jainism Oxford Paperback Encyclopedia 4.