I'll go point by point...
Why are some questions in an orange background on the main page...
The posts with the orange background indicate that those questions contain tags that you have listed as your favorite tags. See here for an in-depth explanation of all the colors on Stack Overflow and what they mean.
How can I get a list of the newest questions for all my favorited tags combined
You are looking right at it! On any view containing questions (ordered by date of course for "newest"), questions containing your favorited tags will be marked with an orange background.
One thing to note here is that we are talking about favorite tags - the system does not only show posts with your favorite tags - it simply marks them with the orange background.
If you would like to see questions only of a specific tag,
you can simply click on the tag itself - eg: facebook. The link will take you to
http://stackoverflow.com/questions/tagged/facebook and from there you can again sort the questions by "newest" to get the most recent questions. From this view only questions containing additional favorite tags will be highlighted, otherwise all the posts would be marked orange rendering the "highlighting" functionality redundant.