Painting is a great way to add character and color to even a rough surface, like plywood. However, you may not be satisfied with a basic paint job, in which case something more stylized, like a wash, is a possibility. The whitewashed look is a popular one, and is simple enough to do with materials around the house. You may need to do a little experimenting, however, as not all acrylic paints will work the same.
Pour a small amount of paint into a paint tray, and add the same amount of water to the tray. Mix well with a stirring stick.
Paint a piece of test plywood with your test batch of whitewash and a paintbrush. After a minute or two, wipe away excess paint with a rag. If you do not have any plywood to test on, you can use one side of your plywood sheet, assuming only one side will be visible.
Examine how washed your mixture looks, and decide whether you are satisfied with it. If the wash is too watery, add more paint, and if it's too thick, add more water. Keep track of how much paint and water you are using, so you can maintain the ratio when you add more paint and water to the test batch.
Add more water and paint to your paint tray, being sure to maintain the paint-to-water ratio that you settled on. Create enough of your whitewash mixture to do the entire painting job.
Continue applying the whitewash, painting a section and then wiping away excess with the rag. The wash will dry quickly, so be sure to stop often to wipe away excess from previously painted areas. When the entire area has been covered, allow it to dry thoroughly.
Seal the paint with a water based polyurethane, if desired for added protection of the surface. Depending on how you plan to use your whitewashed plywood, this may not be a necessary step.