You don't say how long you will be in Rome but if the Vatican and Colosseum/Forum are the only attractions on your list, then don't buy the Roma Pass; it doesn't cover the Vatican. And don't use a 3rd party for your tickets: as Chani said, buy them from the OFFICIAL websites.
There is no single ticket for the Vatican, and only so many areas tourists are allowed to access:
Vatican Museums/Sistine: ticket or guided tour/ticket
St Peter's: free
Gardens: mandatory guided tour/fee
Necropolis: mandatory guided tour/fee
Scavi: mandatory tour/fee (reserved on a different website and limited slots)
You may purchase all but the scavi tour here:
http://www.museivaticani.va/content/museivaticani/en.html
http://www.museivaticani.va/content/museivaticani/en/visita-i-musei/scegli-la-visita/visitatori-singoli.html
Most tourists only visit the museums and the basilica, and tours are available on the Vatican website for both. Unless one is a serious museum lover, I often recommend the 3-hour combo tour of the museums/Sistine/Basilica as it covers just the highlights of a vast collection, and is an efficient way to access St. Peter's directly without dealing with its security line, as you would if entering the church from the outside. This tour is also economically priced.
Entry tickets for the Colosseum/Forum/Palatine should be purchased here:
http://www.coopculture.it/en/the-colosseum.cfm
General-entry adult tickets will cost you 14 euro (12 euro + 2 euro reservation fee) and cover entry to all 3 sites. As you'll see on the website, tours and rental audioguides are also available to book/purchase.
Palatine and Forum are considered one attraction and must be visited together as the sites are adjoining; you pass from one into the other inside the secured area.
(Sorry for the redundancy, Donna; was still typing as you were posting)