1) In this sentence, "regras" could be either "rules" (as in "laws" or "precepts"), or "patterns," or even "models." Only the context will make it clear; however, I'm inclined toward "patterns."

The original text is also somewhat clumsy, as suggested by the following literal translation: "Here are represented the centers of the patterns, the ones assigned to the group, with a value…"

Depending on the rest of the sentence, you could achieve a (slightly) smoother English rendering this way: "Shown here are the centers of the patterns that were assigned to the group and that have a value…," or "…and whose value…"

2) "...consi[s]tui a fracçao da cota (nivel de cinzento) do grupo" = "…constitutes the proportional share (i.e., the gray level) of the group…" I'm not completely happy with this suggestion, but it seems to fit the facts, unless the somewhat looser "...constitutes a proportional share of the [collective] gray levels for the group..." would work better in context.

(Semantic footnote: Black, as a color, doesn't admit of shades; therefore, any color that is a "shade of black," as opposed to "pure" black, is actually gray. Fussy, aren't we? -- Cheers, HC)