Emissivity of Flat Black Paint

Matt black paint emits within a few percent of a perfect black body, which makes it the working approximation for ε = 1 in radiation calculations, calibration targets and heat-sink finishes. What surprises people is that colour in the visible spectrum has almost nothing to do with it: at ordinary temperatures thermal radiation peaks around 10 µm in the far infrared, and virtually all paints — white, black, red — emit at 0.85–0.95 there. A white-painted radiator sheds essentially as much heat as a black one.

Where colour does matter is solar absorption, which happens in the visible and near infrared. A white roof reflects most incoming sunlight while still emitting strongly in the infrared, and that combination — low solar absorptance, high thermal emittance — is what makes cool roofs work and why selective surfaces on solar collectors are designed for the opposite pairing. When you see one emissivity figure for a coating, check which spectral band it refers to; the single-number habit hides the most useful distinction in radiative heat transfer.