Scrape off as much of the paint as possible with a spoon or some other blunt metal tool. If the paint is dry and old, it may flake off with a little encouragement. If the paint is still wet, blot the area with a paper towel to clean off the paint.
Cover the paint spot with a drop of liquid moisturizing soap. Let the soap sit for one or two minutes, then wipe it off the leather with a damp cloth.
Add a small amount of leather cleaner to a sponge. Use a product specifically designed to clean leather surfaces. Squeeze the sponge, let the cleaner foam up, than pat it over the stain. Let it sit for a minute, and then wipe it off with a paper towel. The chemicals in the cleaner may break down the paint and allow you to wipe it off. Check the directions on the product for colorfast testing. You may wish to apply the cleaner in an inconspicuous area first so it does not fade the color of your purse.