Diarrhea does help to cleanse your colon as well as flush out toxins.
So when you're sick with the flu, stomach virus or ate something that caused diarrhea then the diarrhea is actually doing your colon and body some good by flushing out those toxins and cleanses your colon.
But you wouldn't want to induce diarrhea just to cleanse your colon and you have to be very careful with diarrhea as having too much diarrhea can dehydrate you.
So if you're having diarrhea you need to make sure you're staying hydrated by drinking some water or other hydrating liquids.
You can cleanse your colon much easier and safer using some colon cleanse medications such as colon cleanse pills, colon cleanse powder etc.