Color change of marker in top right corner histogram

When changing the exposure slider in CameraRaw (plug-in and filter), the histogram in the top right corner of the window changes dynamically. When a certain number of pixels end up at level 0 or 255, the color of the triangular marker changes.

Is there any publicly available reference to the conditions under which the color change happens (such as, for example 0.5% of the pixels of the channel are there)?

Thanks in advance for any hint.