Yes, you most certainly can paint the exterior of your pool. Pretty much any color you'd want. First wash off any dirt/debris and then use some sandpaper or wire brush and brush off any rust that may be occurring. Next you'll want to prime your pool wall. Get a good quality metal primer paint. I suggest rolling it on with a fine nap roller, so you don't leave nasty brush strokes. Or if you are good at it, and have one handy, a paint sprayer works the best. Let primer dry overnight...now apply your color of choice top coat..something in a good exterior enamel for metal surfaces again. (Rustoleum has some great products)...apply this coat, and let dry. Lastly, spray or roll on a top coat of flat or satin clear finish.
It'll be good as new, and last a long time.
Dan