It s just a generalized description Sim. All fabrics will be transparent if thin enough, but black materials less than white, due to the way they can absorb and transform the visible radiation to heat. Then white thick cotton is better than black thick cotton. Not if you want the convection, then black should be what you aim for. White reflects light of all wavelengths whilst a red cloth absorbs all other colors than red better, how good it absorbs those other will be seen as how red the cloth will be to you, also depending on the light-source of course. Black also reflects all colors, just like white, but as it absorbs so much of them the net effect becomes a black. That I didn t mention it before was because I was trying to keep to what the main question was why do people wear black in the desert.
But you re right, it bears on the question under the caption. It also have to do with the type of eyes we have. They don t separate by wavelength only, they re not built on that principle it seems. Our brain take the information received from the eyes which it then interpret. That s the reason why yellow can be seen both with the frequency reflected being yellow
or be perceived as being the same with a equal blend of red and green, both saying yellow to us. The retina contains three types of color receptor cells, or cones. One type, relatively distinct from the other two, is most responsive to light that we perceive as violet, with wavelengths around 420 nm; cones of this type are sometimes called short-wavelength cones, S cones, or blue cones.
The other two types are closely related genetically and chemically. One of them, sometimes called long-wavelength cones, L cones, or red cones, is most sensitive to light we perceive as greenish yellow, with wavelengths around 564 nm; the other type, known as middle-wavelength cones, M cones, or green cones is most sensitive to light perceived as green, with wavelengths around 534 nm. Light, no matter how complex its composition of wavelengths, is reduced to three color components by the eye. For each location in the visual field, the three types of cones yield three signals based on the extent to which each is stimulated. These amounts of stimulation are sometimes called tristimulus values. There was a study in the 80s about this.
The results were clear. As the report puts it: The amount of heat gained by a Bedouin exposed to the hot desert is the same whether he wears a black or a white robe. The additional heat absorbed by the black robe was lost before it reached the skin. Bedouins robes, the scientists noted, are worn loose. Inside, the cooling happens by convection either through a bellows action, as the robes flow in the wind, or by a chimney sort of effect, as air rises between robe and skin. Thus it was conclusively demonstrated that, at least for Bedouin robes, black is as cool as any other colour. The Bedouin are an Arab seminomadic group, who have historically inhabited the Arabian and Syrian Deserts. Edit: I m not sure if any current research has bunked this.