Use acid free materials. This means you should be using one of these brands of mat boards, Crescent, Bainbridge or True Vue Miller. Check labels on all other types of boards. Lay the piece to be framed out flat on the table on something white. Look away from the piece for a few seconds then look at it again.
Chances are fairly good whatever it was in or near the center of the piece.
That might be a good choice for the color of the undermat, that's the mat closest to the piece. The function of the undermat is to draw the viewer's attention to the central focus of the piece. This might be a border or an object near the edges of the piece. This would be the likely candidate for the top mat. Now place both mat samples on the piece in the order that you intend to use them.