I think that your fabrics will work beautifully (listen to Sewingpup's advice on the flannel), and I'm with Iceblossom's first comments on this. The joy of a pattern like Trip Around the World is that there is very little that won't work. If you do a quick Google search, you will see all sorts of variations on tone placement and they all look just amazing - it all just depends on what you like. If you do the light to dark thing, you get sort of an ombre effect; if you mix your tones evenly, you get something that is reminiscent of four flights of stairs going to a central point. If you alternate lights and darks, your quilt can look a bit like one of those crocheted afghans. trust your instincts and enjoy the adventure! :-)