Quality of Light
Light transforms a room or space even more than paint does. The sun moves around all day, casting shadows and hitting the walls from different angles. We consider sunlight's effects on our antiques, wall art and cushions, but sometimes we forget about the effects on the walls themselves.
All day long -- while we're at work, or dropping off and picking up the kids -- the spaces in our homes are getting hit by light that can transform color in just a few years. Estimating how your colors will fade means you can include those changes in your plans.
Consider also the emotional impact. When creating a breakfast nook perfect for early-morning coffee, for example, consider what happens to that space at night. What was a bright, airy space this morning could become stark and naked, lit by just a chandelier. Giving this space a softer pastel, instead of monochrome white or gray, helps it serve double duty.
Or think about a dramatically lit family room that gets less light during the day. While a darker, more exotic color may lend excitement to the space at night or when you're entertaining, by day that room will be a dead spot, no matter how well-lit. Think about the room's daily life cycle, not just your single-use ideas, and plan colors accordingly.