Similar Values Associate
When you look at this quilt design it is easy to see that there are red wheels and yellow wheels. Even though they don't actually join up the reds associate to form wheels and the yellows do likewise because similar colors associate.
At the same time similar values associate. All the reds are dark and all the yellows are light.
This second quilt design is scrappy. The association between colors is not strong. There are several greens, for example, but they don't combine in any significant way.
However there is a strong association between colors with a dark value and a strong association between light values.
Successful scrap quilts depend on the association between value more than they do on the association between color.
This last quilt is all about value. Even though there isn't a strong value contrast there is enough contrast for the wheels to be seen.
However, this quilt is more about texture than anything else. There is no strong design or pattern.