I don't know much about colitis, but I have IBS and can have similar issues with fiber. What I've found out is that there are two types of fiber: soluble and insoluble. My IBS is very sensitive to insoluble fiber, but I can tolerate a decent amount of soluble fiber: www.livestrong.com/article/27561-list-food
In addition to seeing a dietician, I would recommend doing some online research. Noone in the medical profession ever spoke to me about the different types of fiber; I figured this out on my own after doing a fair amount of web-searching.