Pardon the newbie question. What do the red corners on some pages mean?
That they have been modified perhaps?
Exactly, those denote local page overrides. For more information, see here.
Edit: depending on what you’ve changed, you might prefer to make those changes in the corresponding master page. Or, perhaps you inserted empty pages at the start of the layout? Worth knowing that inserting extra pages “using” an existing master page is still considered a page override - you’re inserting a page that starts with a specified template, rather than assigning a master page change to a page (which is the green marking).
So when inserting a title page I should not insert a new page first and then change that new page with a masterpage template I made? Should I apply the title page template instead on the first music page to avoid overrides?
You can do it either way - I’ve written before on the forum that sometimes, in order to “see the steps” of the process in action, I insert a blank page, assign my title master page to it, then remove the override (leaving the master page change behind). You can also simply assign your title master page as a master page change directly on page 1, and Dorico automatically shifts everything along - the First page of music gets shifted onto page 2 in the layout.
Whichever way makes sense to you, or makes you feel most secure, is fine 
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Thanks @Lillie_Harris. Now I even know how to remove my mistaken overrides.
One day I have to sit down and read the whole manual from start to finish!
I wouldn’t necessarily recommend that, as it’s a sizeable beast more aimed at being used as a reference. I get the impression you’re settling into Dorico pretty well, so you may already be beyond our new First Steps guide but that is designed to be read and followed start-to-finish if that’s of interest.
I’ve added some extra keywords relating to page overrides and their colors to the manual (although that might not make it out until a new version) - we tend to avoid naming colors specifically in the body of manuals as if the developers change the color, it’s very easy for such details to get missed.