Making Switches Work in SSRS
Yeah, SSRS can be a little cumbersome with its handling of empty/null values. Try adding an explicit isnothing() before the yellow option:

=Switch(Fields!DaysToExpiration.Value &gt; 31, "Green",  
    Fields!User_Certification_Status.Value = 8, "Firebrick”,
    Fields!User_Certification_Status.Value = 13, “Orange”,
    Fields!User_Certification_Status.Value = 11, “Orange”,
    Fields!User_Certification_Status.Value = 10, “Orange”,
    Fields!User_Certification_Status.Value = 12, “Orange”,
    IsNothing(Fields!DaysToExpiration.Value), "Gray",
    Fields!DaysToExpiration.Value &lt;= 30, "Yellow", 
    1=1, "Gray")