Hrmm this article here makes it sound like you can also do this with a mirror, but it implies that not any mirror would suffice, and that they had to design a special mirror surface to get one that also radiates heat at a wavelength that will let it escape the atmosphere:
> The first layer is reflective silver. On top of this are alternating layers of silicon dioxide and hafnium oxide. These layers improve the reflectivity, but also turn the mirror into a thermal radiator. When silicon dioxide heats up, it radiates the heat as infrared light at a wavelength of around 10 micrometres. Since there is very little in the atmosphere that absorbs at that wavelength, the heat passes straight out to space. The total thickness of the mirror is around two micrometres, or two thousandths of a millimetre.
> Fan puts the installed cost of mirrors at between $20 and $70 per square metre and calculates an annual electricity saving of 100MWh per year on a three storey building