When it comes to choosing a mount for your black and white photos, there are a few things to keep in mind. The most important factor is the overall contrast of your image. If you have an image with high contrast, then a white mount will help to make the blacks appear even blacker, and the whites appear even whiter. On the other hand, if you have an image with low contrast, then a black mount will help to bring out the details in both the shadows and highlights.
Another thing to consider is the tone of your photo. If it has a warm tone (yellow/orange hues), then a cool-toned mount (blue/purple hues) will create an interesting juxtaposition. Similarly, if your photo has a cool tone, then a warm-toned mount will add some visual interest.
Ultimately, it's up to you which color mount you choose for your black and white photos. Just keep in mind the overall effect you're trying to achieve and let that guide your decision.