A color wheel can be a useful starting point for understanding the relationship between colors, but you don't need to be an expert. Here is a quick overview:

A color wheel is divided equally into 12 sections, each displaying a different color according to its "pigment" values.

All colors come from the three primary colors: red, blue and yellow. These colors are primary because you cannot make them by mixing other colors together. They are represented on the color wheel at equidistant positions, forming a triangle. A mixture of the three primary colors produces black.

Mixing equal values of any two primary colors together creates the secondary colors of violet, orange and green. Since violet is a combination of equal amounts of blue and red, it lies halfway between those two colors on the color wheel. Orange lies halfway between red and yellow, and green between yellow and blue. The secondary colors form another triangle. 

This is the easiest color scheme to understand and work with. By using several different values of the same color you will be guaranteed a harmonious and classic look. For example your hall might be a light taupe, the family room a medium brown and the dining room a rich chocolate. Many color chips are gradated this way from light to dark.

A complementary color scheme combines two colors that are opposite each other on the color wheel, like yellow and purple, or orange and blue. Usually one of the two colors will dominate, like the blue on this color chip, and the values of the hues can vary. This combination of contrasting colors can be quite lively. A further extension of this is a triadic scheme with 3 colors equidistant apart, like red, blue and yellow.

The color wheel is divided into a warm side and a cool side. Colors ranging from yellow to orange to red-violet are considered warm. Warm colors tend to be stimulating and work well in areas where there is activity and interaction like a sunny yellow kitchen or earthy terra cotta dining room. Warm colors tend to "advance" or "condense" a room, creating intimacy.

The cool side ranges from blue-violet to blue, green and yellow-green. Cool colors are calming and are perfect for areas where you relax like a lavender bathroom or ocean-blue bedroom. Cool colors tend to "recede" or "expand" a room making it look larger.

This is because bright colors, or pure hues like sunny yellow, are saturated with pigment. They contain little or no white, black, or complementary tints to tone them. Saturated colors like yellow, orange or lime green are high energy colors and are often best used as accents to give a visual punch. On a large scale the color intensifies and may be too strong. Conversely, lower-intensity colors can give a room a calming effect. Rooms facing south generally receive more natural light and are able to withstand deeper, more saturated tones. Large amounts of warm bright colors like yellow may be too intense when there is alot of bright light.

No, although white is the standard choice and pre-mixed ceiling paint is a toned white. Ceilings, often referred to as the fifth wall, can really be any color. Painting the ceiling a lighter shade of the wall color will reduce the contrast between the wall and a stark white ceiling; this creates a more unified look and keeps the eye from focusing on the ceiling line. For really high ceilings a color slightly darker than the wall will make the room feel cozier. A contrasting ceiling color can have great dramatic impact.

It depends on the lighting and the atmosphere you want to create. Rooms facing south generally receive more natural light and are able to withstand deeper tones or cooler shades. On the other hand, in a dark room on the north side of the house, warm hues with a light value may work better. Contrast also contributes to making a room look larger or smaller. High contrast between furniture and walls will make a space look smaller whereas similar values and related colors will expand the room. Remeber that cooler coolers recede, pushing a wall back and warm tones advance.

Have you ever picked up a neutral beige color chip at the paint store and by the time you got home it had somehow turned green, orange, or pink? The paint chip didn’t magically transform but is the casualty of a color phenomenon known as Metamerism.  This can be defined as the visual difference between two color standards when viewed under different light sources.  It is highly possible for two colors to appear identical under incandescent light, and yet appear completely different from one another under fluorescent or daylight.  

Philip Reno, from G&R Paint in Seattle, explains that almost all commercially available paints are formulated with only 2 to 3 pigments.  Manufacturers “in order to minimize the metamerism between their paint chips (not made from real paint) and the actual paint, formulate colors with extremely simple pigment structures. These formulations then have a tendency to look flat (two dimensional) and lifeless when we put them on the wall.” 

The reality is that all paint colors will change under differing light sources. People often want to choose their paint colors in “natural light, where there should be an optimum balance between warm (yellow undertone) and cool (blue undertone). But even natural daylight is not consistent from one day to the next, or from morning until night.

The ideal situation is to test actual paint samples, or look at chips made from real paint, in the room that you are planning to paint.  Paint companies like C2 strive to make colors that have depth and complexity using many pigments that embrace the beautiful altering effects of light.