While other grading labs make use of similar terms, there are slight differences, like the omission of the term “vivid” by HRD lab. If a diamond has a higher depth of color than Z, it normally falls under fancy colored yellow. Canary diamonds are the most priced among yellow diamonds.
The yellow color comes from the presence of nitrogen in the stone. During formation, some of the carbon atoms are replaced by nitrogen, and this eventually creates a difference in how the stone interacts with light; it takes in the blue and violet parts of the incident spectrum and lets out the yellow part. It only takes very little nitrogen to cause significant coloration in the stone, which is why yellow diamonds are fairly common, as you would know if you ever tried buying a 1-carat yellow diamond ring.