With asteroid 2012 DA14 passing within 17,000 miles of Earth next week on February 15, now is a good time to look at an interest way that could potentially be used to deflect inbound asteroids to safer trajectories. I know about gravity tractors and mirror arrays, but not spray paint; it goes to show that sometimes seemingly simple solutions can be used to tackle complex problems.
In 1902, Ivan Yarkovsky suggested that the daily heating of an object rotating in space could exert a small force on the object. We know how the temperature on Earth gets warmer and warmer as the Sun rises, hitting a maximum somewhere in the early afternoon. The same thing occurs on an asteroid. When the heated surface of 1999 RQ36 points its hot afternoon side in the direction of its motion around the Sun - the escaping radiation acts like a tiny rocket thrust, slowing it down and sending it closer to the inner solar system. Although tiny, a little push day after day, year after year for hundreds of years can change an asteroid’s orbit significantly – transforming an Earth impactor into a close fly-by or a clean miss. (source)
So by spray painting a certain part of an asteroid or all of it, either with light or dark paint (depending on what you want to do), you can modify its trajectory and gently, oh-so-gently, push it into a safer trajectory. This isn't the kind of technique that would be used when there's imminent danger, but it could be used as a preventive way to nudge near Earth objects away from the only planet we have.
David Hyland, a professor of Aerospace Engineering A&M University in Texas, and his team are trying to get funding to do a test of this technique in 2012 on the 99942 Apophis asteroid which has an orbit that brings it pretty close to Earth.
WIRED has more details.