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Some of the purple leaf plums would remain relatively small and look nice against the white background. They flower, but produce only tiny hard fruit that is not messy. You may also wish to consider a fairly tall growing shrub such as choke cherry. In some places they can be a bit invasive however. They would provide a nice green, shiny-leafed foliage that would look nice. The transpiration from the leaves would also provide a little cooling effect, which is always welcome in a summertime machine shed. Some of the bradford pear varieties don't get very tall, and do provide some wonderful appearing blooms in early spring. I would include some evergreen species to provide some color year around. Some yew shrubs would be good, however be aware that some are poisionous. How about some sumac to provide brilliant red fall foliage? Pampas grass, zebra grass, or a clump of the purple giant foxtail would really add some color. Perhaps you would even consider "espalier". Espalier is the training of a plant along a trellis, wires, or other support. Weld up an interestingly shaped trellis and away you go.
Dwarf apple trees are often used. http://www.appleart.com/ A little more involved is the process of producing "circus trees". See the linked page.
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