Short answer: yes, most park benches can be painted later if you change the color scheme, but the prep work matters more than the paint itself. I have repainted both wooden and metal benches, and the results really depend on how the bench was originally finished and what condition it is in now.
If the bench is bare wood or has an old weathered finish, you can usually repaint without too much trouble. Lightly sand the surface to remove flaking paint or old sealer, clean off dust and mildew, apply an exterior primer, and then use a high-quality outdoor paint or solid stain. For metal benches, check for rust first. Remove loose rust with a wire brush or sandpaper, prime with a rust-inhibiting primer, and then paint with a metal-rated exterior enamel. If the metal is powder-coated, you can still paint over it, but you need to scuff the surface lightly so the new paint can grip.
The tricky ones are plastic or composite benches. Some can be painted with specialty spray paints designed for plastic, but they may chip faster in high-use areas. Always test a small hidden spot before committing.
One more thing: if you are changing from a very dark color to a light one, expect to use extra coats and a good primer. If you plan to paint later, avoid wax or oil-based sealers unless you are willing to sand them all the way back.
So yes—save the color scheme change for later if you want. Just remember that preparation is the real secret to a long-lasting finish.