There are some simple reasons for this. First off, there are many colors that appear good initially but their appeal changes to a great extent when they are applied to the walls. Different factors such as light affect the over all look of the walls and sometimes you have no way of knowing until the paint is on and dry.
Another thing that should be kept in mind while choosing the colors is that darker colors make the room look smaller and lighter colors make the room look more spacious. This can be used to your advantage by making ceiling height appear higher or rooms seem longer.
