Have you heard of a Gray Scale and Value Finder? I use it every time I make a quilt. It prevents many fabric mistakes. It is printed on card stock, found at art supply stores and still costs less than four dollars.
Starting with 100% black, each rectangle increases white 10% and decreases black 10%. The lightest rectangle is 10% black, 90% white. While they are numbered 1-10 there is no consistency between manufacturers whether 1 is the darkest or lightest. Just lay out fabric, place the value finder on top and squint. The fabric has a value (percent black or white) where it fades into the value finder. For example, my carpet is a 2.
We frequently hear that value is more important than color. The Gray Scale helps quilters track values across all their fabrics. Trading blocks is easier too because you can refer to the value scale (assuming everyone in the group has the same brand.) No, they don't work for every fabric. A large scale print with lots of color may have different values in different places.
The first one I owned had small holes made by a hole punch but these larger openings are better designed. The Color Wheel Company manufactured my current value finder. Other than a fondness for their products I have no relationship with them. Let me know how it works for you.
Knitters should consider this too. I saw color and black-and-white photos of gorgeous Fair Isle knit sweaters. Sometimes the pattern (and all the hard work) disappeared in black-and-white because yarn color changed but not value.
Fret not; enjoy the day.
Ann