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.

Before and after matting applied. Colour sampled from the cat's eye.
To replace transparency with a colour matte:
  1. From the Edit menu, click Matte.
  2. 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 primary colour from the Colour panel.
    • Secondary Colour—uses the currently set secondary colour from the Colour panel.
  3. (Optional) Reduce the Opacity to limit the colour strength.
  4. Click Apply.