I think you will find that, no matter what the tartan, both black and brown accessories will 'work' well - but not together, of course!

I have black belts, sporrans, boots and shoes. I also have the same items in brown leather, but the colour choice is usually determined by where I am to wear them. As Colin so rightly says, black tends to be a little more formal than brown.

When I was a lad (I just missed the Dinosaurs!) black was for evening wear, whilst brown was for day wear, along with the tweed jacket which would never be worn after dark!!

[Oh, and I also have both black and brown leather 'bomber' jackets to complete the picture!]