Double matted - I consider something double matted if there are 2 layers under the primary focus of the element. If it's a really neat sticker, Jolee item or even a brad, then 2 colors under it is double matted. I tend to do more double matting of "things" on the page rather than the photos, but it all depends. But I can't think of a time I'd ever use 3 colors! That's just too much IMHO!
When I use CM pages - or any plain, white page - I almost always double mat my photos. Otherwise it's too bland. However, I like to wallpaper. Then I just mix it up & tend to double mat special or my favorite photos on the page.
I recently did some heritage recipe albums for Christmas gifts. I did wallpaper the pages, but they were a combo of CS & PP. For photos/elements on the CS I used some of the PP as one of the mats and tended to double mat. For the things on the PP I used only one CS mat for the most part, but I was using a Fiskars lace corner rounder - so there was added interest there.