Prep can be a pain but just wait it out. Take plenty of clear liquids. Once it turns yellow and runs like water, thats perfectly fine, that's just the bile in your stomach. Patience is the key, as is keeping yourself hydrated. All the water you take in now will just help you flush it out more easily.
The brownish stuff you see is actually the coating for the intestines. Usually it's just mucus but time after time of passing stools will have coloured it. Like I said, patience.
And good luck with your colonoscopy!
Hopefully you're getting knocked out for it, but if you're getting just sedated I suggest asking for gas and air as well, just as a backup.
Love, B xxxx