Found these links for what it's worth - hope they're useful x
Looks like some say high-fibre, low fat diets, others advocate raw meat. If you feel that you can research and provide a complete raw diet adequate to your cat's needs from what they say on some of the sites that might be the best way to go, otherwise I'd say at least go for the hypoallergenic foods.
Hopefully now I've bumped this 4 u someone with experience of colitis in cats will come and offer you some better advice! but for now, your reading material x

Colitis in Cats-Causes, Symptoms & Treatment - Cat-World
Raw Meat Diet for Cats - Pet Debates Forum - GardenWeb
Harpsie's website - feline colitis and IBD