The way it looks above is not the finished product, it got so much more added to it the colors started mixing and almost looked a little purple in some spots, but we made sure they didn't mix too much. After all, it was supposed to be Patriotic, and there's no purple on the flag!
All you need is a white poster board, and red and blue washable tempera paints.
The idea is to let your little one squirt the paint on the poster board, and walk/hop/skip/run/crawl all through it. They can get a new sensory experience by squishing the paint around with their toes. You can talk about why red, white and blue are patriotic colors. Most importantly, it gets kids up and moving and creates a wonderful work of art when it's all said and done.
Note: Doing this outside like we did will help minimize the mess. It's also handy to have wet wipes nearby and possibly towels ready in the bathroom for a quick rinse off.