i'd do it using the stamp tool. make the radius quite large and but i'd only have it on 50% strength perhaps. then i'd fix it on the center of one of the red squares in the bottom, and i'd fill in the white square at the bottom using single clicks, not long sweeps. go over and over a few times, switching the origin of the stamp tool to the red square on the other side so that it doesn't look too uniform. and then pretty much repeat for the rest of the image.
there might be an easier way by using a layer style and tinting the white to red perhaps?