Nothing adds as much to a room as paint does. There isn't a design element out there that gives you as much value and return on investment as a simple can of paint. The first step in decorating any space should be painting. Your room can go from blank canvas to evoked emotion in the few hours it takes to paint the walls. And that is before you even bring the furniture in. Before you paint your room, you want to consider what emotion it is you would like to evoke. An overview of the colorwheel and its colors will give you some guidelines to follow, though these certainly are not hard and fast rules. In your home, anything goes.

Warm colors.
Red, Orange, Yellow. This part of the color spectrum suggests warmth, passion, coziness, hearth and home.

Cool colors.
Blue, Green, Purple. This portion of the wheel gives feelings of cool calmness and serenity.

Where the warm colors are loud and energetic, the cool colors are quiet and introspective. But, because most colors, particularly the ones you might find at the paint store, are a blend of colors, you can have warmer or cooler versions of hues. So there are cool purples that are more blue than red, and warm purples that are more red than blue.

Taking into account the room you are working with and its primary function, you can choose a warmer or cooler color. A cozy study could use a warm color like a warm rust; a home office might work well with a cool, crisp green. It all depends on the desired effect you are looking for.

One additional thing to keep in mind is that no room exists in a vacuum. You can have each room in your home with a completely different feeling, but you may have to consider the flow so that the transition from room to room is not jarring.