[surfsvammel]

No. This is not a good idea. You are not supposed to eat stuff. The whole point is to empty out your system. It would not be great to be half way through the procedure when they say: “oh. Here is a bunch of M&Ms, we can’t see anything. We need to redo this procedure in a week. And this time don’t eat stuff…”

[Smaug123]

(As well as making sure you don't have actual matter blocking the view, you also want to avoid staining the bowels with something that could be mistaken for blood; so also don't drink red stuff.)