Colleague asks for feedback on a paper of which I am a reviewer
Have you considered writing a fresh answer to the colleague without caring that you wrote (or are writing) a review? What I mean is, just write the answer on your knowledge of the paper without caring that you are also writing a review. I think this is a natural approach and does not really have a downside.

You know the paper quite in depth, and you can provide valuable feedback. You will do so (or already have done so) in a formal way through the peer-review system, but now you have the opportunity for formulate the feedback in a more casual way (compare it with writing down results for a formal publication in a journal or at a conference and a discussion of the results on your personal blog - you describe the same things in a different way of communication). The only downside I see is, that the person will get some more clues that you may be a reviewer but I am not sure if this really is a downside at all.

Another downside may be that it costs you more time and that you don't want to allocate time to provide feedback twice. In this case, you can politely answer something like "Thanks for the paper - I try to find time to read it soon and come back to you in case I have any feedback." I guess most people understand that this can also mean that they won't hear anything more from you on that matter…