Matting is a useful feature which allows you to swap out regions of transparency with a color of your choice, particularly useful when repurposing transparent PNG files.

Before and after matting applied. Color sampled from the cat's eye.
To replace transparency with a color matte:
  1. From the Edit menu, click Matte.
  2. From the dialog, choose one of the following:
    • Custom Color—pick any color from the adjacent swatch using a pop-up HSL Color wheel.
    • Primary Color—uses the currently set primary color from the Color panel.
    • Secondary Color—uses the currently set secondary color from the Color panel.
  3. (Optional) Reduce the Opacity to limit the color strength.
  4. Click Apply.