Do you have a color wheel? Once you pick the "Definitely this it the one I'm gonna use" fabric, it is the baseline for the decision.
If you want "blendy" like you indicated, then you have two choices:
1. You can choose fabrics in different values/patterns of the same color.
2. You can choose fabrics that are on either side of that color on the wheel.
You could go for contrast by going opposite on the color wheel and pick from each of the two colors to make up your selection.
There is also a triad where you split the difference on the color wheel and pick a total of three colors.
Hope that helps.