Harmony refers to elements arranged in relation to one another that make the composition well-balanced. Unity refers to the holistic quality of an art composition.
Harmony in color can be achieved through the minimal use of colors and not too many opposing colors. Value in art refers to the lightness and darkness of a color.
When similar shapes or forms are utilized in patterns or repetitions, it creates a sense of consistency in the composition, which creates a sense of harmony.
Texture in art is created by brushstrokes; if there is a consistent brushstroke style applied in a visual composition it can give it a sense of harmony due to the rhythm or flow the strokes follow.
Color and Value:Bridge Over a Pond of Water Lilies (1899)by Claude MonetShapes and Forms:Mont Sainte-Victoire (1904 - 1906) by Paul CézanneTexture:Olive Trees Under a Yellow Sky, and the November Sun (1889)by Vincent van Gogh