i use whatever seems appropriate at that time. if i piece horizontally or vertically, however, i stay with the same fabric. if i try to change fabrics, it's too hard to keep the seams exactly lined up with the finished edges. on a large frame on a queen quilt, if the top shifts even 1" then the seaming on the backing doesn't match the selvedge line of the top anymore. what i mean is it could end up not squaring at the corners. i don't know if i described that well.
anyway, if i plan to use more than one color, i use them diagonally. that way, nothing looks wonky.
pinwale (baby) corduroy is yummy. don't use red. even prewashed.