A mini-lesson by Nancy Chong
Value is the lightness or darkness of a color. In the quilting world, value means the relative lightness or darkness of a fabric. The word relative is used because the value of a fabric is not absolute, but is always gauged in comparison to another fabric. Below is a photo of fabrics arranged from white to black (A to K).

Only white and black have a definite place on the value scale. Everything else could be classified as light, medium or dark when set with other fabrics. In the photo above, Fabric E is a medium. In the first photo below it is a dark, and in the second photo below it is a light.
The difference in the value between two pieces of fabric is called the contrast. Black and white are very high contrast, since they are on opposite ends of the value scale. Placed next to each other, there is no doubt about where one ends and the other begins. Black and gray are close in value so they are low contrast as seen in Fabric I below.

Prints can be classified as low contrast or high contrast. A low contrast print has only one or several similar values occurring on the same piece of fabric. A high contrast print has more than one value occurring on the same piece of fabric. What you see above are low contrast fabrics, prints that read as essentially one value. Below are fabrics that are high contrast fabrics, because within each piece of fabric there are light, medium and dark values in the print. Your eye will almost always read yellow as a light value, regardless of what a value finder might tell you.

If you make pieced quilts, you can see that using the fabrics above would be a challenge whether you needed a dark, light or medium piece. No matter where you cut, a different value would appear along the seam line.

The low contrast pinwheel on the left uses all fabrics with the same values.
The only thing that really stands out is the yellow flower. On the right,
a higher value (lighter) background automatically creates more contrast, but the
green and purple fabrics also sharply contrast with
each other and with the background. Every point stands out crisply.

The low contrast star on the left uses a print fabric with several values
and colors. Again, the
yellow shouts for attention. The pale blue fades into the medium value
background and makes
the right hand point nearly disappear. On the right, the print has
been moved to the background and the value of the fabric is much higher than
the fabrics used in the star. A dark purple batik
with tiny yellow gold dots is interesting without overwhelming everything
else.
The same in true with appliqué. Appliqué quilts are all about value. Every appliqué piece you stitch should show off against its background, whether that is the background for the entire block, or the appliqué piece that lies beneath it.

A single blossom becomes more dimensional with proper use of values

Changing the value of the background and other overlapping pieces creates
more interest and allows your design to stand out