The narrow outer ring that scores double the segment number is called the double ring in darts.
Each numbered segment on a standard dartboard has a small outer scoring band. A dart landing in that band scores twice the number printed on the segment: double 20 is worth 40, while double 1 is worth 2.
The double ring is crucial in x01 games such as 501 because a player normally has to finish on a double. It is distinct from the treble ring, which lies nearer the centre and multiplies the segment value by three.
The outer bull is a different area entirely. It surrounds the central bullseye and has a fixed value rather than doubling a numbered segment. These rings are identified by position and scoring function, not simply by their colours.