That's why Roy G. Biv, owner of Rainbow Painting in Brush, CO, makes it a point to understand the basics of color and how perceptions of color affect his work - and his customers' expectations. For example, it generally is true that light colors make a room look larger and dark colors make it look smaller. It's also true that wet samples generally dry a little darker, since the sheen reflects more light. Further than that, though, you'd better not go. When you're asked for an opinion, you have to get some information from your customer before you can give it.
Different colors have different effects on people.
Although nothing absolutely definitive has come from them, there is enough evidence from which to draw general conclusions, however. Blues and greens are supposed to have calming effects, for example, while reds evoke dynamic responses in people. But if for some reason you don't like blue, it isn't going effect calmness. You can make some general statements about the psychology of color, but you have to look at each individual as just that, an individual with individual preferences. More light is reflected by bright colors.