For the artwork to recede, the top mat is the darkest value, and the undermats get lighter as they near the picture. For the artwork to come forward, the top mat needs to be a lighter value than the artwork and the undermat near the artwork should be considerably darker. A color will seem to change depending on what color is next to it. Purple will appear more blue when placed next to red. Purple will appear more red when placed next to green.
Value is also changeable.
A color that appears dark when surrounded by white will appear much lighter if surrounded by black. The reason is that any color put on a mat changes the appearance of the colors in the artwork. Try different combinations of matboard next to your own artwork, or ask your local art store or picture framer for suggestions. A shade is a color grayed to make it a darker value.