Here's the deal with shade cloths according to  and previous references in that:
- Black shade cloth is only about 50% efficient. In other words, a 50% shade cloth will only block 25% of the solar radiation/heat versus no shade. The reason for this inefficiency is that the shade cloth absorbs and then reradiates the energy.
- White shade cloth is almost always better than black shade cloth, even at lower percentages of blocking (up to a limit) -- e.g. you'll do better with 40% white than 55% black.
- More important for plants, but still relevant: the "shade rating" for shade cloth (white or black) appears to be based on the photosynthetically active radiation transmissivity, not solar transmissivity. i.e. the shade rating is applicable mostly only to the visible spectrum, not the remainder (heat, UV, etc.).
- Black shade cloth is not effective without some sort of active cooling mechanism (e.g. airflow). Wetting the shade cloth helps tremendously, because the water evaporates, taking the absorbed heat with it. Consider using a misting machine of some sort if you use black shade cloth. At least ensure you have wind or other active airflow to remove the heat.
--- I would at least buy white shade cloth, and not bother with the black. Aluminet is probably best, but it's expensive. White and black shadecloth both benefit greatly from being wet.
 "The Effect of Cloth Characteristics on the Cooling Performance of External Shade Cloths for Greenhouses", D.H. Willits. J. agric. Engng Res. (2001) 79(3), 331}340
"The essence of tyranny is not iron law. It is capricious law." -- Christopher Hitchens
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