Is the prayer of a colon patient who suffers from excessive flatulence valid, given that the prayer posture changes from standing to sitting when there is a fear of passing wind?

Your prayer is valid, and you are not required to continue in one of the two positions. So, if one is afflicted with incontinence while standing but not while sitting, then he should pray sitting. It is permissible to use allusive language if you are embarrassed to disclose this matter.