WHEN A WHITE DRAGON IS NEUTRAL (Article 105)

Commentary: The dragons correspond to the numbered suits: Dots with White Dragon (i.e., blank, soap), Bams with Green Dragons, Craks with Red Dragons. The White Dragon can also be used as a zero in a block of year tiles. In this case, it no longer corresponds to the Dot suit. 

Question from a player:  If I am playing a year hand where I need a White Dragon, must the number tiles be Dots? 
 
Answer from the NMJL:  The White Dragon is usually a tile with a blue border around the edge. This is often referred to as “Soap.” Only this Dragon can represent a zero. The Soap, when used as a zero, can be used with any suit. It can be used with Dots, Bams or Craks. 

Source: Mah Jongg Made Easy (2024 page 5 paragrpah 2)

“White Dragon when used as a zero does not have to match the symbol tile it is used with.  White Dragon as a zero is neutral” 
 
Other answers from the NMJL:  “The [number tile] may be played in Cracks, Bams, or Dots.  A White Dragon, when used as a ZERO, is considered to be neutral and therefore can be used with any suit.”  
 
Source for other answers: 

National Mah Jongg League Bulletin Q&A (1983 page 4, 1984, 1991 page 12, 1992 page 5, 1993 page 13, 1995 page 5, 1997 page 6, 2014, 2015, 2018) https://www.nationalmahjonggleague.org