1. Not considering light
Without light, color wouldn't be much to look at. And as much as the right light can make a color sing...it can also make a gray go lavender or a white look dingy. Before painting a wall, use a sample painted or taped up to view your potential new color in all light of the day — as well as artificial light. If a color you thought would work doesn't, take it back to the paint store and explain to someone working there which type of light made the color go wrong — someone with the right experience should be able to point you in the right direction.