create a guide. place two horizontal and two vertical guides to line up your groups where you want them. for more information read rulers and guides. hide the guides and rulers when you are done.
create a new layer (⇧⌘n). using the flood fill tool, fill it with white. place it below all of your 'corner' groups.
now for the fun part, adding color! click on one of the ‘couple’ layers within a group in your layers list. choose filter ▸ color adjustment ▸ replace color. click on the 'color to remove' color well and choose white. click on the 'color to replace' color well and pick your color from the colors 