Mats are more than decoration, they protect the surface of the art and keep it from laying directly on the glass. To decide which mats to use, look to the colors in your art for inspiration. Using multiple mats will add to the beauty of the finished piece. Here are a few basic rules:
Dark colors tend to contain the art, while light colors tend to expand it.
Mats should be at least twice as wide as the frame width. In a multiple mat combination, that means the top mat.
Vary the amount showing on each mat in multiple mat combinations to avoid “striping.”
The top mat can be whatever color is predominant in the art. You can also use a neutral top mat with lower mats as accent colors.