Do I need to wait after having a colonoscopy before I can donate plasma?

Yes. Having a colonoscopy is safe, but some patients can be dehydrated afterwards. In addition, there is a risk of low grade fevers after the procedure. Waiting a few days is all that is likely needed. At that point, if one feels well and has no signs of illness one can donate.
Yes. You should wait at least a week after a colonoscopy to hydrate yourself and also to limit the infection risk in the plasma.