For all math nodes you need to know that poser is treating each color internally as number between 0 and 1. So i.e. white = 1 and black = 0. And therefor colors can add, subtract and so on. RGB colors are treated channel by channel. So the RGB 0 ... 255 values refer to a poser 0 ... 1 value.

You may add or subtract value to a result under 0 or over 1 but keep in mind that these results can not be seen directly but as the nearest "in range" result (BB is calling that hypercolors). E.g. Black minus White minus White is equal to 0 - 1 -1 = -2, so logical a super-super deep black but showing only as normal black color. With this in mind it's not surprizing that adding only once white to this result will stay a black color cause -2 + 1 = -1. Otherwise like adding black and white 0 + 1 = 1.

But really it's great to work with those nodes. They are essence in texturing. But try starting from easy steps. BB's nodes are the essence with a look to most possible reality but in a lot of cases you can find solutions with less (processor time consuming) nodes. For example if a medallion is ending as a 10 x 10 pixel great piece in a 3000 x 3000 pixel render there is no need for a super realistic shader tree for gold color. So the complexity is even a matter of individual circumstances.