Rome: Guided Tour of the Colosseum Underground By Get Your Guide is who we used 2 weeks ago.
They were fantastic, we were the first tour at 8:45 and visited the underground first, then the rest of the Colosseum. We spent approx 90 minutes at the Colisseum. We then went to Palatine Hill, where we walked around and took in some stupendous views of the Roman Forum with the Colosseum in the background.
I would definitely recommend this tour. Hope this helps.