Since you've decided to switch to a black and white palette, then grey would be your medium. Of course, value is relative to what it's compared to. Cream is light until you put it next to white; then it's "dark"; put cream between white and brown, and it's medium.
A white-grey-black ordering of fabric would certainly be easier to determine value, but ... be sure to look at scale. When you chop up fabric into small pieces, you want to be sure you capture the whole color you're going for. Prints that are too big will have large expanses of color and you won't be able to guarantee every piece you cut is the color you want. So pick some solids, tone on tones, small dots, checks, geometrics, florals, paisleys, etc, that will show up in each piece you cut.