This week’s mailbag article about pink snow, http://www.straightdope.com/mailbag/mpinksnow.html , states:
Cake frosting? I could’ve sworn it was the Cat’s bathtub ring.
[[Cake frosting? I could’ve sworn it was the Cat’s bathtub ring.]]
And the bathtub ring came from the cake (frosting) the cat was consuming before he let out the water. Duh.
Jill
Not necessarily. If I recall my CITHCB correctly (and I should, having read it to my kids any number of times over the past year), it was a “big long pink CAT ring” (emphasis mine) and “looked like pink ink”. NOT a cake ring.
They do indeed call it a “cat ring,” but it is pink like the cake he was eating in the tub. Why would a cat leave a pink ring? The kids were mad at him for eating the cake in the tub, and when the water ran out, voila, there was the ring. Let’s be realistic here.
It only looks the same as the frosting because of the ancient printing techniques in use when the book was published. There were only two colors available, blue and red, and light versions of each. It was impossible to adequately capture the subtle differences between “pink frosting” pink and “cat ring” pink. 
[[It was impossible to adequately capture the subtle differences between “pink frosting” pink and “cat ring” pink.]]
Please elaborate on your theory about the etiology of the pink cat ring. I have to say I feel for you, having read this book so many times and apparently totally missing the core gist of the book. The bathtub ring was the feared but expected outcome, which is why the boy was aggitated at discovering the cat eating cake in the tub. Jeez, the things I have to explain to people around here…