I'm a Type 2 on a Low Carb way of eating. I have regular colonoscopies (for a genetic problem causing polyps).
Although a low reside diet includes lots of carby foods, it also includes many things that I normally eat:
Officially - Cauliflower (florets only), Chicken breast, eggs any way but NOT fired), normal cheese (such as cheddar), fish.
Thing which are digested before the large colon (my brother has a stoma bag and confirms): well cooked (tender) meat without skin, including beef, pork, lamb and turkey, soft cooked veg (asparagus tips, beetroot, green beans, spinach, squash).
Definitely no seed, nut or whole grains, or fibrous veg.
I usually only stick to this for 2 days before fasting for the remaining time, but that is personal choice - instructions only say to fast overnight for next day appointment.
Good Luck with your colonoscopy.