Explanation to the experience

In spite of there being no way you can obtain 'pure' black color, you can mix diferent colors which then together seem black! Amazing, huh??

Well, the next question might be: "why don't the colors spread as black and separate instead?"
Like everything, those 'colors' are made of particles which wheigh diferently. So, lighter colors like blue are taken further away with the water when heavier colors like brown settle down earlier.