I'm honestly curious. I've seen it in Digimon and (allegedly) in BoBoBo-Bo-Bo-BoBo. Is this some kind of Japanese censorship thing? Is it just a stylization? I mean, what the actual poo?
How else would authors do all the ice cream jokes with it? (see e.g. Arale making ice cream in "Dr. Slump") Jokes aside, while I am by no means an expert on the topic, I'd guess japanese culture being somewhat obsessed with cleanliness and freshness makes it less unacceptable to display what comes out of the disgestive system in non-natural color.