Thank you for the double matting idea. I played around with it and so happens gray inner mat and white outer mat works very well because the photograph is split toned. The gray inner mat picks up the tone/color of the subject's clothing. (it's a gentle portrait of a mother holding her baby) It also made an interesting repeat of colors. Strangely enough, it looked the best when I didn't leave too much whites outside of the dirty border..... I didn't expect that.
Thanks everybody, too, for ideas.