There is a right side and a backside to each fabric whether the color is the same or not. If you look closely at quilting weight cotton you can see the warp and weft threads on one side more clearly than on the other (shows on the back of the fabric most generally. Also there is sometimes a difference in the sheen of a fabric between the front and the back. It doesn't matter which side you put facing up but if you mix them you may be able to notice it when the quilt is all put together. I often piece print fabrics with the wrong side up because I like the look of it better than the front of the fabric in with my other colors. Good question!