I have had Ulcerative Colitus for almost 20 years, its not there all the time. I get attacks about once a year (never during pregnancy), it won't be cured, its always in the background, just sometimes it can hide for quite a while.
I think there could always be a risk with anal sex whether one has colitis or not. That I would mean for both partners.
Ulcerative colitis starts at the anus/rectum...the very thought of having anal sex while having UC (which I have) gives me a "pita".
Colitis meaning inflammation of the colon could mean anywhere in the colon, not necessarily with the anus/rectum involved.
If it is an interest, I would strongly suggest always using a condom (so that the transmission of anything doesn't go either way) as well as lots of lubricant as to not cause any physical tearing (which could set one up for other problems if the anus/rectum is already compromised from inflammation.
It's all a matter of perspective!