Oh, it's random. When I cut the strips I put the fabrics into stacks of 4, just seperating the blues from the browns. I then put the stacks of 4 blue on one side and the stacks of 4 brown on the other. 31 different fabrics cut in stacks of 4 gave me 4 stacks of blue and 4 stacks of brown. I randomly paired a blue with a brown.
When I have the pairs of squares cut from the sewn strips I'll lay them out just to check that I have an even distribution of light and dark.