Look through your stash for small pieces of fabrics that have lots of light in them. With the dark pieces they will read as lights. It's okay to purchase some if you need it, at least you are using those darks and making something useful out of them. Look through your closet for "light" garments that you no longer wear in fabrics suitable for quilting, or your husband's white/light shirts, pillowcases, mismatched sheets, tablecloths you no longer use, etc.