Which category does your business fall into? Are you the kind of person to overlook appearance and go straight to work, or are you the kind that sees the value and importance of both?
The magnetic lasso tool allows you to select specific parts of the clipart. The result is that you don't have to color the entire image the same color. Magnetic lasso properties can be altered to fit your needs. However, you must be familiar with the lasso tool in your particular program.
Some designers actually make their available online in its original format, so that you can download it and make some changes, as well as resize it, though most will only supply a high resolution version that can be used up to a certain size poster. A Google search for.svg clip art will give you vector art you can use in Inkscape, and in fact some of it is exceptional.