That is great information but irrelevant to the pictures in question because the rotors pictured are almost certainly carbon/ceramics. Only the GT2 class of ALMS requires steel brakes.
For example: cast iron is dull gray ("colorless") at 1000 F but bright tomato red by 1500 F.
True, color rendering is extremely hard and changes drastically with variations in light, however photographers can control the color rendering in their pictures through controlling the white balance. Also, as an fyi unless you have carefully calibrated your monitor you are not seeing the colors as they are meant to be displayed.
And in variable, uncontrolled lighting conditions, judging color with true accuracy is very difficult if not impossible.
That is quite a broad assumption that you are making there. I think these pictures challenge your assumption that the hydraulic pressure is uniform across the face of the rotor. The reason why this pressure and therefore temperature and in turn color varies is what we are trying to understand.
We know that disc braking sytems are carefully designed to apply large amounts of hydraulic pressure uniformly across the faces of the rotors. And since brake engineers are pretty damned good at what they do, they largely accomplish this.
I cannot speak for the other photographers but I can guarantee that the pictures I have taken and uploaded were taken in heavy brake zones at Sebring (either turn 3, turn 7 or turn 10) which means that the driver was almost definitely not varying their pedal pressure in a way to cause this effect. I also took some video with my camera this year and it clearly shows these color variations and the differences in the variations car to car are also obvious.
So when we see the rotors glowing in a uniform pattern across the face, or perhaps even center-hot, we can safely presume that the driver is applying moderate to maximum pedal force. This is a thoroughly understood, carefully and reasonably controlled process. However, when the caliper pressure is decreased -- as when the driver trail-brakes or steps off the pedal altogether, for example, this controlled process ceases, and a far less controlled (at times even seemingly randomized) process takes its place. It can look very dramatic and variable, but the actual variations in temperature are relatively small, and thus their causes are relatively subtle.
While this is a quite ridiculous video I found, it shows how the brake disc can glow at different temperatures at the same time: