Cheapest and smartest way to purchase tickets to Vatican and the Colosseum?

I'd like to take advantage of my time remaining for my upcoming trip and book all my museum tickets online in advanced for a discounted price.

I've been researching so much regarding tickets to the vatican and the colosseum, but I can't seem to get anywhere with which tickets I need to buy for the best price.

I'm looking for a non-guided tour of the Vatican which includes skip the line, but I can't seem to find it on the official website of tickets.museivaticani.va and can find it on the third party ticketing website of thevaticantickets.com.

Ideally, I'd love to purchase from the official site, but I can't seem to understand if the official site sells skip-the-line passes for the vatican.

Same would apply to the Colosseum. Where can I book my tickets (including underground ticket) officially?

There seems to be a lot of third-party websites disguised as the official site.