code: private static Collection<TileRef> getTiles(Map<ColorRgba, Collection<TileRef>> colors, ColorRgba color)
      {
          if (!colors.containsKey(color))
          {
              colors.put(color, new HashSet<TileRef>());
          }
          return colors.get(color);
      }
docs: Get the tiles corresponding to the color. Create empty list if new color.
  
  @param colors The colors data (must not be <code>null</code>).
  @param color The color to check (must not be <code>null</code>).
  @return The associated tiles.