You can significantly improve the appearance of an older above ground pool by repainting the support walls. But you can inject additional style to your yard by color-coordinating the pool to match any outdoor color schemes, or camouflaging it with natural color so it visually recedes into the background of the yard. You can also paint design motifs on the pool to give it even more character.
Examine the type of material used to form the outer support walls. If constructed of flexible polyvinyl, such as is common in inflatable pools, you will need to do something other than paint the walls to reliably change the color. Thin polyvinyl does not accept paint very well. Even special paints designed for use on polyvinyl will tend to crack and peel off as the inflatable walls flex with the normal use of the pool. In addition, some paint products designed for polyvinyl can interact with UV rays and cause thin polyvinyl fabric to degrade faster. Some pool manufacturers will void any warranty if thin polyvinyl walls or liners are painted. You can, however, build a soft "skirt" around an inflatable pool and change the color or look that way. Sew a number of fabric panels in double widths, with hem pockets along the top and bottom edges. Manufacture as many of these panels as needed to fit all around the diameter of the pool. The original pool color may show through the "cracks" between panels now and then, so keep this in mind by color-coordinating the fabric with the blue or green of the inflatable walls. Create two long elastic bands using seamstress elastic: make these bands long enough to fit around the diameter of the pool edge snugly. Thread the fabric panels through the top and bottom hem pockets onto these elastic bands. Install this giant "skirt" over the pool. The elastic bands and the fabric will give with the movement of the pool.
Apply paint to other types of solid plastic walls. Solid polyvinyl will "take" paint without as much cracking and peeling. Use an enamel spray paint intended for polyvinyl and other plastics. Avoid using smaller cans of spray paint purchased at a local home improvement store because this method is very inefficient. Use of a commercial-sized paint sprayer will take less time and result in a smoother finish. You can often purchase "plastics" paint in large quantities from auto-body paint shops. You can also rent a paint sprayer for the day, just be sure to rent a unit intended for use with oil-based paint products.
Paint aluminum or wood sidewalls using oil-based epoxy paints. Epoxy paints can be purchased in gallon and quart sizes, and in a wide selection of colors and finishes from home improvement stores. Use a spray gun designed for oil-based paints for the best finish, or roller brushes. Use smaller "sash" brushes, rated for use with oil paints, and artist brushes for any detail work. Marine epoxy has the highest rating for endurance of the elements. You can also seal the finished work with a coat of oil-based urethane.