I had to do that, too, prior to a colonoscopy. I don't know if they forbid red colored foods - I couldn't eat those due to it looking like blood in your GI tract, I guess, but did see that Jello was allowed on my list of things - obviously not vegan!
I did recently discover a vegan pseudo-Jello made by Hain, though. It has dehydrated cane juice, colors derived from vegis & carageenen from sea weed. The "wobblers" recipe is pretty good (like Jello wigglers). I believe that they call the product "Fruit Squeeze" or something like that. It comes in a box like Jello & you add boiling water & put it in the fridge to set.
Vegi broth, apple juice, etc. are always possibilities, too. Good luck with your tests!
It is better to be hated for who you are than loved for who you are not.