Matting is a useful feature which allows you to swap out regions of transparency with a colour of your choice, particularly useful when repurposing transparent PNG files.
To replace transparency with a colour matte:
From the Edit menu, click Matte.
From the dialog, choose one of the following:
Custom Colour—pick any colour from the adjacent swatch using a pop-up HSL Colour wheel.
Primary Colour—uses the currently set foreground colour from the Colour panel.
Secondary Colour—uses the currently set background colour from the Colour panel.
(Optional) Reduce the Opacity to limit the colour strength.