I am not a scientist so we shall leave it to a simple explanation of why they are this way. As the plant is stressed (eaten, squashed, uprooted or gets diseased or old), the chlorophyll in the pads disappears and the various other elements that have been obscured begin to show through. Each decaying pad seems to have a different pattern and its own beauty.
We start off with a mature and healthy pad.
There are holes.
In many cases the dominant color goes toward pink
Finally the skin of the pad begins to erode to reveal the understructure
just as in the saguaro.