Answer: There are a few common cactus pests that fit the description you've given, so you'll need to put on your detective cap, take a magnifying glass, and get up close to your cactus. If the affected areas are bronzed and mottled, the likely problem is spider mite damage. If the patches are corky, it could be due to physical injury, insect damage or sudden chilling. Brown soft patches indicate a rot disease, and hard brown spots can be scale insects.
Based upon your observance that the color is draining out, I'd suspect scale insects. These common pests attach themselves to stems (or in this case pads) and suck the juices out of the tissues. The result is a pale-looking area where the scales are feeding. Try dislodging the spots. If they pop off, they're probably scales. If the spots won't peel off, and affect the tissues below the skin of the cactus, they may indicate disease.
After you've done a little detective work, let us know the results (via this Q & A platform) and we'll provide some treatment options.
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