I drank strawberry milkshake when I was on clear fluids only before a barium meal/swallow test. I knew full well it wasn't allowed, but I was pissed with the doctor because I thought he was wasting time repeating tests when I'd already been tested so much and didn't really care if the test worked or not. They didn't notice; the test went fine, they got nice clear pictures of everything they needed to. (And it
was a waste of time - didn't tell me anything I didn't know already.
)
The instructions will always err on the side of caution and clean patients out as much as it's safe to do so. Everyone will clear out differently, due to many factors such as disease state, previous diet, response to the prep, medications, variations in natural function of the digestive system, etc., but they give the same standard instructions to everyone, with the odd exception, such as having had large amounts of intestine removed. The standard instructions aim to include even those most difficult to clean out, so for many, not following them to the letter - consuming something that's not a clear fluid, not managing all the prep or bringing some back up - will still result in being adequately clear for the test.
So unless you're very unlucky, I think it should probably be ok.
And if your doctor didn't even mention avoiding red jello, they can't be overly worried about it.