I use dark and light colors within the same color family (example: yellow-greens instead of blue-greens) to give my quilts a bit of a scrappy look. If all of your fabrics that are too much alike, it seems like there isn't any reason to change fabrics.
If you want to add in a darker (or lighter) fabric than the main prints you're using, make sure you pick a few of those pieces so that that particular shade doesn't stand out.
The main thing to do when choosing fabrics is putting them all together to see how they look together. With some practice, it'll soon become second nature! Good luck choosing your fabrics for your next quilt!