Hello and welcome to the forum. You don't mention what you are currently feeding but on the whole, it is probably best to choose a product that has a very simple recipe and is easy to digest. One of the good quality wet foods would meet this criteria. If you need help using the filters to source a suitable food, please ask. If you wish to use dry food then it might be easier for the dog to digest if you soak it first. Also remember to give at least two meals per day so that the dog's stomach is not empty for any length of time.
Dogs can be intolerant to various things but common ones are wheat, chicken, beef, dairy, eggs. I recently heard of a dog that was found to be intolerant of potato. However, any one of a number of ingredients could be the culprit so that is why I suggest that a trial on a food that is very simple in recipe may be a good place to start.
Colitis can be due to food intolerance but sometimes no cause is found. You should seek the advice of your vet if you want further investigation into this. Often the vet will prescribe an elimination diet and the dog will start out on a food that has hydrolysed protein e.g. Royal Canin, Hill's. The owner then slowly adds different proteins to gauge the dog's response. You can do this yourself using one of the wet foods that has only one source of protein in it. However, the trial needs to be very gradual and you would need to read up about this. Naturediet has some fact sheets on their website that might be helpful but there is plenty more information on the Internet.
You might also want to consider a raw diet. Complete raw meals make this quite easy. Raw tripe is said to be good for dogs with colitis. The other type of food that is worth considering is a cold pressed product
due to their digestibility.