Ah! colon cleansing is the MAIN thing everyone must do to restore themselves to vital health. :) The colon is KEY, so yes, do the P&B shakes, colosan, whatever feels right to you. And, since it's the main path of elimination of all toxins, it behooves you to keep it clean and moving as you do any other kind of detoxing. Otherwise, the toxins sit there and get re-absorbed back into the bloodstream, free to stress the liver, lymphs, skin, all over again!
Fiber tends to push everything thru the stomach and small intestine very quickly, so it must be taken alone, either 30 minutes prior to meals/supplements, or 1 hour after. Whether it's daytime or nightime doesn't really matter, altho some people do find they can handle the fiber easier duruing the day when they're more mobile. Movement/walking helps the colon push the stuff along. ;)
Definitely start learning about colon cleansing! Dr. Bernard Jensen 's book is a quick, easy and highly informative read.
If the P&B shakes make you constipated, don't worry, I have a long list of natural cures for that. ;)