If you're set on creating your own outdoor cinema screen, then there are some materials that are better than others. If you have a very flat and even wall outside, then you can paint this pure white. If you want to get fancy, try painting a darker border around where you want your projection to be.
Otherwise, blackout cloth is your best option. Color options include white, light grey, and black, with each achieving a slightly different effect. Blackout cloth is more durable than many other options, and it absorbs ambient light rather than reflecting it.