At first glance, the concept of tone may seem “vague and insignificant”; but it is one of the most important concepts in art. It seems easy, but it is difficult in practice. Why? Because “tonality” is an optical phenomenon; and, as any optical illusion, is perceived as “not real”. Our brain does not acknowledge it. For the human beings, color is important (we choose clothes by color; we judge fruit by how ripe it is, etc.), but noticing the tone often seems useless and unnecessary. We assign significance to distinguishing the green traffic light from the red, but we absolutely don’t care which of them is darker and which is lighter. It does not even occur to us to compare.