Kath, here is the official website for buying entry tickets or tours DIRECTLY from the Vatican:
http://www.museivaticani.va/content/museivaticani/en.html
This is the page of the different options:
http://www.museivaticani.va/content/museivaticani/en/visita-i-musei/scegli-la-visita/visitatori-singoli.html
Take some time to look it over and give a shout back if you have questions?
For the Colosseum, this is the official website:
https://www.coopculture.it/en/the-colosseum.cfm
A general-entry ("open") ticket also includes entry to the Forum and Palatine. It doesn't cover guided tours; those are extra.
As the other stated above, Viator is only a re-seller - a third party - and doesn't provide tours or guides directly themselves. There's a LOT of discussions in the recent forum history about tickets, tours and whatnot if you do a search for them.