Yes, a cold can certainly set off a flare. In fact I am experiencing that right now, I was almost in remission, then today I woke up with a cold and bam, symptoms returned this afternoon. Grr. This has happened the last three times that I have had a cold.
The sooner you can get rid of the cold, the better your chances of your UC calming back down. I reccomend boiling some sliced ginger in water and then strain it, and squeeze in the juice of a lemon. Sweeten with a little raw honey if you want to.