I am also an admirer of Janice4's work!
About adding more fabrics, consider that if you feel you need more interest or more "movement" within the quilt, then you could use fabric that has more in it, frequently tone on tone or sprinkled with an "off" color, which might be good for an old truck. Stitching could deliniate your truck's doors and door handles and even tread on tires. What Janice said is so true, just go with it without rules ('cause there aint none).
Play with fabric scrapes, or not, and with threads and with the different stitches available on your machine. You've got a fabulous start!!!