By default all layout mats are white in color.

1. You can change this by applying a template style with a different mat color.
2. You can also add bevels to a layout using the same method.

However, in order to make the bottom & right edges of any bevels (which are pure white) easy to see, all light coloured mats are automatically displayed on the screen slightly dimmed down.

For general Display purposes, people who don't use bevels and use high-key photographs in their layouts and prefer a pure white mat, there is an option in the preferences (Display --> Borders & Backgrounds -> Layouts) that allows the user to select whether they display constantly dimmed layout mats or "Only dim white mats with bevels". (This was added as of v3.2 and newer).

If you use high-res production, when creating a composite print,  ProSelect applies the same slight darkening of a white mat for the same reasons using the default darkening of the mat color slightly by 2%.  The "Darken White mats with bevels by:" setting can be changed or set to 0% (off) in the Preferences area (Hi-Res Production --> Style Options tab -> Composite Prints).