When sounds are more than one, they begin to interact with each other. When we play the notes together, they form a "harmonic interval". Interval may sound consonant or dissonant, which is known as consonance and dissonance. Euclid gave this definition: "Consonance is the combination of two tones, high and low ones. Dissonance, on the contrary, represents the inability of two tones to combine, and that is why they sound awkwardly".
From the physics perspective, dissonance is caused by beating between phase difference of sound frequencies. For example, the frequency of 440 Hz lags behind the frequency of 460 Hz by 20 fluctuations. When two tones with small frequency differences (i.e. <20 Hz) are presented simultaneously, one frequency with beating amplitude is perceived. This sound abatement is called beating. The sound with 20 to 60 beatings per second is perceived as dissonance.
Consonance, on the contrary, does not have clearly expressed beatings; therefore sounds supplement each other, and are perceived as a harmonious structure.
One octave has 12 intervals including unison (two tones of the same pitch). Three intervals - minor third, major third and perfect fourth, form when combined correctly the consonance chord. The major or minor sounding of a chord is determined by the order of these intervals.

Why the diagram represents less colours than the rainbow colour count?

Why a note can not correspond to a colour?

Why the intervals are to be given the numerical values of semitones?

Why the interval of 3 semitones is compared with the blue colour and not with yellow one, for instance?
How the multitude of colours we see is expressed in music?

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