Harmony: something experienced as being together. Harmony may be consonant or dissonant.
Dissonance: the discomfort or clash of two or more ideas, materials, or frequencies of light or sound.
When I place two elements close in time or place, for example two colours, two sounds, or two words, a vibration arises between them. We feel this on a scale from beautiful to ugly.
The difference of each individual's experience of art, in all its forms, keeps it vital, dynamic, and alive.