i tried mounting a project on formcore and didn't like it, the core is much too soft and the images did all sorts of unlikely things.

as to the backs being the same color as the window.
Many people cut their own windows and buy board in large amounts and so one has the board handy and available.

On the other hand, we have used different shades of white as the back board if the window overlays the print, as it will not show.
If one floats the print within the window, it visually looks better (IMHO) to have the back board the same as the window.
Just a personal opinion