When I see an asteroid barreling towards the Earth ready to smash into it, I don’t think “Hey, let’s paint it white! That’ll save us from impending doom.” Here’s the concept, though:
On a hot sunny day ideally one would wear a white shirt, because it reflects the most solar radiation instead of absorbing it like a dark shirt does. With that same idea in mind, if you paint part of an asteroid white, the colored section will feel more push from solar radiation. That push would gradually nudge the asteroid off course and away from colliding into Earth. It wouldn’t necessarily need to be paint, either. You could use dust or chalk.
The point is that it has to be white and it has to cover a portion of the asteroid so that it can be pulled off course. So, next time your friends get in a debate on what to do in case of an Armageddon situation, dazzle them with the idea of painting the asteroid.