What we really need to consider are those 30% that are exceptions to this rule.
In the spring and summer when you are wearing white flannels, white twills, white poplin or sailcloth slacks or seersucker, you will want to wear your white bucks. White suede belts, however, have no place in a gentleman's wardrobe. White fabric belts may be worn with trousers of exactly the same shade of white as the belt only. Hard white trousers with a hard white belt work because the belt becomes, essentially, invisible. White belts are never, ever, worn under other circumstances. Better than white belts are woolen twill belts or whimsically embroidered d-ring belts, all great for summer wear. In circumstances like this you will likely wear a sport watch with a metal band or a fabric band which may be coordinated with your belt. Coordinating your wallet with the other leathers is a very elegant touch, but not necessary. When wearing fabric alternatives to leather, a plain brown or black leather wallet is perfectly appropriate.
Another example of the 30% where the rule does not work perfectly: In the current era, black and navy slacks are often worn with brown shoes to excellent effect. I really enjoy this look as a more casual application of trousers that are typically perceived as dressier. The rule of thumb would dictate that with the brown shoes or boots, a brown belt should be worn, but that is not always the best. With black slacks, for example, a light or medium brown belt puts a stripe at the gentleman's midriff, which is not always the most flattering look. A dark brown belt may blend with the fabric colors better, but if that is not an option, break the rule and wear a black belt with the brown shoes.
In short, use the rule as you see fit. Matching leathers coordinates an outfit well, and gives a sense of finish to it, but it absolutely not a hard-and-fast rule. Keep it in mind, but break this one at will.
