Why can’t I have red jello before my colonoscopy? My doctor said any clear liquid is fine, and red jello is clear. So what’s wrong with it?

Red . Red (and purple) jell-o and other clear liquids with these colors can look like blood in the colon and also absorb the same wave length light that is needed to highlight the sometimes subtle color differences in the colon that we need to see in order to identify the pre-cancerous polyps that we are looking for when performing colonoscopy. It can make it more difficult to see these polyps, increasing the chances that something is missed.
Color. There may be a specific reason your doctor has but often people worry the red color can be confused for blood. And then they would worry they are missing important patholoy.