There are a number of theories put forward as to why such outer wings were completed in grisaille , with the most common being that they echoed in paint the tradition of covering religious imagery with grey cloth during the then 46 day lenten period leading up to Easter .
There are a number of theories put forward as to why such outer wings were completed in grisaille . 
 The most common , first advanced by Molly Teasdale Smith , is that they echoed in paint the tradition of covering religious imagery with grey cloth during the then 46 day lenten period leading up to Easter .