I have always been advised to be in bright/light colours during the trekking expeds in regions which are known for bad sun.
Everybody knows that Black clothing absorbs more heat(radiation). The lighter you wear, the lesser heat you attract(radiation). Now there is a point rightly said above that the darker colours will emit it faster as well, but the media for emission is very important. We, human, emit the body heat in the form of sweat (usually, the most of the times). After a certain amount/threshold, Sweating is trouble-making. The more you sweat, the more body-necessary fluid you loose rapidly.
The physics, or I should say, the physiology of that is something like this:
Perspiration is our major Thermoregulation mechanism. The more you exert (Specifically, out in the sun, not resting), the more your muscles heat up due to exertion, more sweat is produced, the more fluid you loose. How the sweat is produced? When the gain of the hypothalamic feedback system increases in response to the increased body temperature, the more Perspiration is needed. And, sweat contains majorly Water, then Minerals, may be Urea (hence Salt??)
In that case, the darker your clothes are, Higher is your skin temperature, and so higher is the Perspiration rate.
Now, addressing to the example you have posted above, it says that, the Namaqua chameleon regulates the temperature with its color, yeah agreed, but there a lot complicated points about its hydrating and body-temperature maintaining phenomena of Cold Blooded animals, typically the Reptiles. You can't simply co-relate their phenomena with our body temperature maintaining mechanism, Because reptiles are Ectotherms, they get their body heat from outside (external environment), and they lack the ability to generate heat in the body, unlike Humans for that matter..
Go for Brighter colours when you plan to trek at any place where there you are expecting the temperature to be above you body temperature by 4-5 Degree Celsius.