Forget the spray paint. Go to your local fabric store and find some nylon/polyester fabric in a bright color. Cut out a large sqare or squares and sew onto your pack. Alternatively you can take some brightly colored vinyl tape and wrap the frame with it
When I was in Uncle Sams Canoe Club I wrapped the handle of my seabag /kitbags in a couple of contrasting colors so it could be spotted quicky in a big pile of green bags