What specifically are you looking for? I think perhaps if you're hoping to find meaning in them, you won't find it. The setts seem to have been chosen by the tartan weavers purely from their sense of aesthetics, and most were assigned to clans on an almost random basis during the Highland Revival period in the 1800s.

I would guess that using black and white to frame in other colours was simply a choice made by whoever was designing the tartan in order to offer a more distinct pattern, or boldness to the sett, which minimizes the colours visually "running together".

As for the primary colours used, I would imagine that this was a function of economy. Green, blue, red, white, black, and yellow are primary colours which are fairly easy to dye consistently in large batches. Given the primitiveness (by today's standards) of the mills at the time, it would have made sense for them to stick with a narrow range of colours, and create the wide variations in tartans by altering the sett, rather than delve too far into different hues.