What are the legal repercussions?

About two months ago, I was on my way to work and was driving hastily(sped past traffic in the left lane going about 5 over posted limit) because the right-hand lane wasn't moving. I then got over and turned off, eventually pulling up to my work. I was followed by some guy in a pickup who blocked me into my parkin, so naturally I got out. We exchanged words, beginning with him telling me how much trouble I'm in for reckless driving, and him taking down my driver's license number. I asked him if he was a cop and he said yes, so I asked to see his badge(He was plain clothes in a civilian vehicle), but he ignored the request. At this point I was pretty angry, because its the second time I've had this happen here in two years, so i said some expletives, and he took off. I realize its too late to do anything legal about this, but out of curiousity if I had reported him, what kind of trouble would he face? I live in Colorado if it matters.