I use an off-white for brown. It's a good transition into that tone and isn't too harsh. I also use it for warm-toned prints. It's a Peterboro PW572. You can go a little more "off" into a beige too and it works really well -that's what I use for my brown/walnut/cedar frames. For straight black and white that are selenium toned to a cool colour, I use a bright white PW521 Peterboro in a black frame. I know this doesn't necessarily apply to your scenario, but maybe you're thinking aobut it wrong. I like to draw the viewer in rather than pop the image out. It's my own preference to stick match with base tint rather than contrast. The image feels more as belonging to the frame rather than escaping it....if that makes any sense.