The choices aren't limited and you can choose any color you want.
Please click on HSB or RGB tabs at the top of the color dialog box to choose a color that is not present in the Swatches tab. I guess other kinds of palette like Benjamin Moore's (or probably a more free one) would be nice too, but in the current version, once you found the color on a palette, you can copy its RGB or HSB code in Sweet Home 3D color dialog box.