It could be that the character has an illness or was exposed to some kind of toxin. The gray might represent a serious condition or just a visual effect for drama. Sometimes in cartoons, these things happen for plot reasons.
Perhaps the character overexerted themselves or was stressed. The gray stomach pain could symbolize a deeper emotional or physical issue. Maybe it's a way for the cartoon to show a big challenge the character has to face.
Maybe they ate something bad or got hurt. That could cause the stomach pain and the gray color could be an artistic choice.
It could be that he exercised too vigorously and pulled a muscle in his stomach area. Or perhaps he has some kind of digestive problem like indigestion or an ulcer.
Maybe she ate something bad and got an upset stomach. Or she could have some kind of illness.
Maybe they haven't eaten for a long time and are hungry.
It could be because they're in a funny or adventurous situation and haven't had a chance to get food. The growling stomach adds to the humor or tension of the story.
Maybe it just ate a big meal and is really satisfied.
Maybe it ate too much food and that's why its stomach is big.
Maybe he got hurt or ate something bad that made his stomach ache.
This is a strange and potentially disturbing situation. Maybe it's part of a very specific story or a way to draw attention to a serious issue, but it's not a common or pleasant theme for a cartoon.
Maybe the kid overexerted themselves or had an accident. That could cause the chest pain.
It could be that she has a stomach illness or got hit in the stomach. Or perhaps she's just really nervous about something.