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Generally speaking, running rich causes only the cat to glow. Lean would cause most of the head pipe to glow as well.
What's the difference? Lean exhaust is hot right out of the head. Rich exhaust is actually cooler. Why does the cat glow? Unburned hydrocarbons burn off at the cat, and if the engine has a PAIR system (air injection) it really ramps up that burn and makes it get very hot.
This was seen a lot in the mid 2000's on diesel trucks that had regen cycles for the cats. The computer on those trucks would ramp up the engine speed to spool the turbo, and do a double injector pulse to get more fuel into the exhaust to help burn off carbon by igniting raw fuel in the cat with a bunch of fresh air. The cats would glow red, and they sometimes got so hot they would melt the cab of the truck or the asphalt under them.
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