Take your picture to a framer where you can test the colors of the mat samples against your picture and your chosen frame.
Select a color from the picture that is lighter than the overall print, but darker than than the wall it will hang on. Find a color that does not compete with the picture, but complements it.
Choose a mat that is a different color and value (the relative lightness or darkness of a color) than the frame. You don't want your picture to appear as if it has one large frame.
Order a matting that will be at least twice the width of the frame. Matting that is at least 2 1/2 inches wide is best for most pictures.
Examine the color of the mat core. A colored core might look better than a stark white core.