Following the scheme of every other new style banner it looks like +2 Brown, +1 Purple, -1 Yellow. I had a post in progress somewhere about the banner specifically and how the new scheme made them easier to intuit what they do some of the time (when you can, for example, determine that it is showing brown and not red, and thats just my assumption because it looks closer to some of the browns on the new banners and the reds are almost all more distinctly red), but harder to differentiate at a glance. Prior to the patch, you had to memorize what the symbols did to know what they all did, but you could instantly tell which banner you were looking at. The new ones have the colors bleed together, are sometimes indistinguishable from other colors, and they obscure the designs.
Anyways, for plus two plus one minus one, the plus two color is displayed on top, the plus one at the bottom, and the minus around the border. For dual plus one banners, the two colors are displayed side by side split horizontally. Plus two pure color banners are all one color.