Natural, bone- or off-white works very well with B&W or colour, but it is a personal choice. A good framer will help you decide on the best complimentary matting. Personally, for more than 26 years I have framed all my Ilfochromes under bone-white 9-ply rag mat with a plain black frame.

I don't really think that websites should guide or otherwise influence your decisions and choices on matting physical prints.