Trastevere transportation confusion

So glad you made it to Italy! We left Rome a week ago but really walked everywhere from our apartment near Campo dei Fiori, including over to Trastevere many times. I'm sorry I can't help but I am sure one of the helpful folks here can. Looking forward to hearing all about your trip!

The metro is the subway and is underground. The tram is on rails above ground and the buses are just buses. :-)
One option is for you to catch the #8 tram at Trastavere Station to Piazza Venezia and then walk past the Forum to the Colosseum.

Or, try this...
http://www.agenziamobilita.roma.it/en/cerca-percorso.html?da=Via%20Ettore%20Rolli&a=colosseo

Are you staying at a hotel where reception staff can advise you? Hotels usually provide extra maps, too. A taxi is always a fall-back option, even though you already paid for the transit pass.

Metro = subway, which is a separate format than tram, train, and bus. Those two Metro lines don't stop right by you, so you probably won't use the Metro much. Choose whichever transport is the most direct to your destination. The transport overview map on pages 22-23 is a good place to start, along with half-way down p. 25, which lists a few buses that are handy from Trastevere. Looking at that map, I would guess that you could take a train across the river, from Trastevere station to Porto San Paolo station, then connect there to Metro Line B for a couple more stops to Colosseo. Stephen's suggestion above for Tram #8 is probably better (and I see the tram to P. Venezia mentioned in Rick's description of Hotel San Francesco, which may be near you). Other maps in the book use the key on p. 2 to identify bus and tram stops.

For more precise point-to-point planning, try http://www.muovi.roma.it/ or their app. See this mentioned on p.20, as well as Rick mentions this video: https://www.youtube.com/watch?v=U7FZFMkR7Os. At your neighborhood train station, the bus stops should have destinations listed.

Generally the metro (subway) is of limited value in Rome. Only two lines that skirt the old city area. Put a business card from the hotel in your pocket. Walking is the best option followed by bus. The hotels always have cheap tear off maps. We get a new one everyday and they are perfect for keeping you headed in the right direction. At the end of the day, you are probably lost (we have been lost in Rome more than once), and you don't want to walk back to hotel. Flag a cab, give 'em the card, and say, "Take me home." Also, from time to time you should see little TIs. Stop in, they always speak English, get maps, get directions.

Finally, for any transportation question. Ask at the front desk of your hotel. They are use to pointing tourists in the correct direction. For on the spot info, they are the best.

I am so happy to hear you made it safely to your destination! As Kate advises, we have always had good luck asking the folks at the reception desk for help/directions. I usually request one of their hotel maps, ask them to circle where we (the hotel) are located and then trace on that map a route to where ever we'd like to go. All have been most friendly and helpful. I have a lot of maps with a lots of directions to a lot of different places! We stayed in Trastevere a couple of years ago and walked to the Colosseum. It's a lovely walk along the Tiber and not all that far.

Flagging a cab might not be easy, but there are taxi stands near every major monument. Enjoy Roma!