Color value is a term that refers to how light or dark a color is, in relation to other colors. Value is an important characteristic because it helps quilters decide how to arrange the fabrics so that they either blend with each other, or contrast against each other.
Black has the deepest value, white the lightest. Colors mixed with black are shades (example: TWILIGHT BATIKS) while colors mixed with white are considered tints (example: SOFTEAK BATIKS), so that a single color, like blue, can be represented in a range of values from the lightest pastel blue to the darkest navy. Lighter value colors tend to "float" or come forward visually, while darker values tend to "sink" or recede.
One way to determine a fabric's value in relation to other fabrics you want to use with it is to view the fabric through a translucent red film or glass. This removes the overtones of any other colors in the fabric, exposing only how bright or dark various areas of the fabric appear.
When something very light is placed next to something very dark, it's said to have high contrast or strong contrast. If a quilt or picture has proper contrast, it's easy to see the design, such as a pieced star motif standing out from the background. In other cases, such as blended quilts, the fabrics are specifically chosen to blend in with each other, rather than contrast.
To learn more about color value and fabric selection, visit our friends at the Quilting section of About.com and read through their color value articles.