Actually the photos are sitting on a white shape with a thin black mat. The tutorial is HERE. It's written for Memory Mixer but the technique is the same for any program. (Background and flower embellishment from Lindsay Jane's Spring garden)
Here's another example with a black background and the outline all around the outside of the tiled photo. I struggled for ages trying to use a pastel background for this one. Sometimes you have to be bold and choose another colour! Add another black shape to cover part of the outline so you can create a gap for the title. My "mat" started out white but then I colour matched it to the cream of the embellishments which are by Dee Bee designs and were free with the Artisan Guild magazine. The page was inspired by this free template. You can use any plain background colour for these pages.