Value is a scale of color from light to dark. Artists use value and often create a value scale to help them learn and apply value.
How can quilters find and use value?
Color Dots on Selvedge
Printed fabric has dots on the selvedge which show all of the colors used during the printing process. These isolated colors can be used to help identify the value within the print.
Value Finder
Use a plastic tool to help find value. Red is for warm colors, Green is for cool colors.
Digital Photograph
An easy way to find value is to take a digital photo of the fabric and then use an app to convert that photo to greyscale.
Photo Editing Software
Open a photograph is software such as Adobe Photoshop. (GIMP is a free alternative). Use the greyscale function to convert the photo to greyscale to quickly identify values.
Intuition
Squint to find value using your eyesight. This is similar to how a camera uses a smaller aperture to modify the amount of light seen.