I take my iron supplements with Vitamin C to help with absorption. I also take Colazal and have noticed no issues.
As a side note, I've been taking iron supplements for probably 9 years now and have had UC for about 18 years. I haven't noticed any issues taking iron with UC. In fact I was in remission for almost all that time until this past January when a bad doc told me to stop taking my meds. I had borderline low iron/ferritin levels before the supplement, now my levels are right in the middle of norm and have stayed that way (until the flare anyway) :)