A very basic guideline for mat selction for photography, is to avoid using a white that is brighter than the brightest white in the image. That is why Pearl White in the Alphrarag line and Photo White in the Alphamat matboard line are so popular. Both are just a little off white but yet a cream.

In the case of sepia toning it gets more complicated since you no longer have any true whites. A couple other general guidelines are use of top mat that matches a color that is present in the image but only a little and that you would like to help bring it out more. Since you have two colors you are concerned about you might consider using a double mat. A couple things to try: Perhaps use Pearl as the top mat, but then use a browner bottom mat board. You can also try a browner top mat that is closer to a brown very prevalent in the image then use a Pearl mat as the bottom mat to help separate the image from the mat since they will tend to bleed together. I just took advantage of the bevel being white on a black mat to get a similar effect in this photo. Here this image is very low key and mostly black and I used black mat with a white core. The white core helps to separate the image from the mat. I could provide a more noticable separation with a white bottom mat.

http://www.framedestination.com/grap...117_silver.jpg

This photo is very high key and mostly white so although the mat board is white I used one with a black core which is the same as using a black bottom double mat:
http://www.framedestination.com/grap...e_502_gray.jpg


Make sure you get mat samples to lay on the image when trying to figure this out.

Cheers,
Mark