 All the interior parts of this nebula have a blue-green tinge that is caused by the doubly ionized oxygen emission lines at 495.7 and 500.7 nm.These observed so-called "forbidden lines" occur only in conditions of very low density containing a few atoms per cubic centimeter.In the outer region of the ring, part of the reddish hue is caused by hydrogen emission at 656.3 nm, forming part of the Balmer series of lines.