An update on painting these things.
Styrofoam friendly acrylic spray paint works well, albeit quite expensive in the arts and crafts shop. Taking a piece of foam to the store to test the melting effect first is probably a good idea. Normal (suitable for brush) acrylic paint works well too.
Going for a lighter colour than the blue one in the styrofoam is hard work. It needs lots of layers to hide the blue tint! This "white" one have one layer of spray paint and two layers brush:
As can be seen, it's still blueish. Looks literally cool. Won't try to go for more of those white ones.. If I ever decide to do so anyway, I'll make sure to prime it first!
Darker colours, on the other hand, works well. This is a mix of bluish green and dark blue with some of the original light blue still shining through:
The styrofoam diffuser project V1 is now done. Have built 2 prime 127, four prime 157, six prime 199 and two prime 211, for a total of 2498 "wells" on 6.4 square meters. The primary goal was to breath some life and euphony back into the listening room, without harming the microscopic precision that comes with loads of absorption. Mission accomplished! The listening room is more fun than ever before.
EDIT: PS! Doh! Forgot the practical information. Sealing the cracks with silicone is good. If painting afterwards, the silicone structure will be accentuated and reliefed, giving the surface a rough look. Painting before applying silicone works better.