Harmony

I personally think the human body itself is harmonious. I used to go through Da Vinci drawings in high school and just look at them for hours. Dancers and athletes bodies memorized me, and do still to this day. It's always baffled me that Americans are so obsessed with body image for that reason. Because regardless of whether you're fit or not, a body is still going to be interesting in it's own way.
Anyway, I digress.
Edgar Degas is one of my favorite artists, mostly because the majority of his art is ballet dancers.

The Ballet Rehearsal on Stage

It's not their faces; most of the time you can't depict the face of the people in his paintings or drawings anyway. It's the movement. Which is ironic, because obvious there is none — it's a painting. Completely still and set forever and onward from the time it was painted 100+ years ago. But the desire to see movement is still there.
I think each of Degas' paintings has a distinct color pallet, which is generally where I see and feel harmony at first glance.

The Dance Foyer at the Opera on the rue Le Peletier, 1872

Compared to "The Ballet Rehearsal on Stage", "The Dance Foyer" is much brighter, but the focal points of both paintings are the dancers. They are all the same colors, but some shaded with different lighting than the others. Despite that, they seem to be the brightest in most of his paintings of dancers. The contrast of the top painting compared to the bottom is more extreme, but it still exists in both.

The blues and grays within the bright white in the dancer's outfits pull them away from the yellow design of the room. The shadow gradually grows darker on the back wall, and in some cases over the dancers, but they still stand out.

During this time period especially, most of the dancers were all similar in shape and size, and their lack of facial detail from Degas makes them lack individual identity, yet they stand alone from the rest of the painting. 

The balance between those contrasts is almost perfect to me, though. The balance of color is what creates the harmony, especially in the top one. And now that I look at it further, I think the grouping of where the dancers are has a great affect on the balance in terms of color as well. 

Often I envision harmony as a bright and warm color, but I think it's better portrayed in the top painting, which is a bit darker. It's still warm from the yellows throughout it, but the green and shadowy background gives it a cool contrast. 

I think I like the bottom one because of the lines. They are primarily vertical, except the bar on the wall and the molding at the top of the room (which is also a nice color contrast to the overall brightness of the piece).

I chose two mainly because I couldn't decide, but also to show how diverse Harmony can be.