Can I add clear flavors to prep drink for coloscopy?

Only if supplied. If you are taking an oral lavage solution, the bolus of liquid you ingest is supposed to wash thru the gut and carry waste out with your resulting diarrhea. However, you can ruin the effectiveness of the prep if some of the liquid gets absorbed by the small intestine before reaching the colon. Using sugared flavorings with your prep will so result in its lack of efficacy, necessitating a re-prep.