Yes. You have three pairs of woofers. Each pair is wired in series, by connecting the positive side of one to the negative side of the other. Then the pair is treated as if it were a single speaker. In other words, you already understand it correctly.
As for the color used on the crossover board, I just wanted them to look nice, since you can see them from the outside of the box. So I just grabbed a can of black spray paint, once the decision was made to paint the cabinets black. Before that--while I was still undecided on the cabinet finish--I sprayed a coat of clear lacquer, but that wasn't really necessary. I just hadn't decided on the finish yet.
As for sizing the crossover board, I position the components on paper first, to get a good idea of the size I need for the board. Curt's idea of using peg board is great. I used some thin plywood scrap I had on hand.