Over time, rusty marks from sprinklers or years of dirt and weather exposure will discolor your foundation. Painting it light or medium gray is a neutral choice that will clean up your home's appearance and provide a clean background for landscaping plants. Bear in mind that very light gray or off-white will show dirt, which is a consideration if you live in a wet, muddy climate.
Choose a dark color, even black, for more impact. A black painted foundation looks very sharp in contrast to a house with stained cedar or redwood siding. Or choose a darker shade of your siding color--for instance, a deep putty foundation with light taupe siding.
Painting the foundation the same color as the siding will make your home appear a bit larger and taller because there's no change in color to create a visual break, which is the way to go if you do not want to accentuate your foundation at all.
If your taste is somewhat eclectic, paint the foundation a contrasting color. Choose something that doesn't clash with the siding--for instance, plum looks a bit startling paired with yellow brick siding, but navy blue looks sharp. Match the siding to the painted trim, or echo the roof color with the foundation paint to pull the house together.