Logic Needed for Turning on the Gray Stoplight

Todd Harrison
edited 12/09/19 in Smartsheet Basics

I am trying to add logic in the following string that will turn on the "Gray" stop light ball if the % Complete cell is left blank. 

=IF([% Complete]18 < 0.5, "Red", IF(AND([% Complete]18 >= 0.5, [% Complete]18 < 1), "Yellow", IF([% Complete]19= 1, "Green", IF([% Complete]19= blank, "Gray")))

I have tried "blank" and "null" and neither one works.  Any of you super users out there figured this out yet?  Thanks.