Nothing grates on the human nervous system like a dominance of dissonance. And so a good sense of harmony is as paramount to a painter as it is to a musician.
Harmony means agreement, a fitting well together and being in tune. Although utilizing similarities is a tool for achieving harmony, it does not mean that everything must be similar. It does mean that differences are balanced so that they fit and are in tune with one another. Look at this painting by Kevin MacPherson.
Differences are made by contrasts. In this painting, MacPherson has contrasted the vertical of the figure with the horizontal of the table as well as the lights along the woman's arm and behind her head with the dark of her hair. He has created harmony by weaving these differences together with a dominance of warm harmonious light and soft, loose edges which keep shapes fitting well together.
In the painting below by Richard Schmid, we see a similar way of achieving harmony.
The contrasts are warm against cool, light against dark, busy street vs. quiet distance. As is his particular mastery, Schmid has finely tuned the cools and warms so that they feel as if they're being lit by the same source. This alone can weave harmony into contrasting elements of a painting. In addition, though, Schmid has carefully crafted the edges so that they fit rather than isolate. How we handle our edges can go a long way toward giving harmony to our work, but being faithful to the temperature of the light source is crucial.
We are all aware of creating harmony by repetition of similar colors throughout such as Pat Weaver has done with the color red in the watercolor below. And we see in Weaver's painting repetition of shape and size and two other harmony-getting schemes.And we're familiar with using analogous colors to create harmony, such as Colin Page has done with blues to greens to yellow-greens in the painting below.
In fact, repeating any one of the visual elements can work as a harmonizing scheme, but to keep that scheme from producing boredom by including too many similarities, contrasts and variations are necessary.
Without a doubt, many contrasts and similarities can be used while retaining harmony within the entire piece simply by carefully crafting edges and keeping the painting's light temperature consistent. And these can be achieved by carefully observing the subject and being faithful to what the eye sees as its unique characteristics.