Add this line to your calculated field 'cor' after the 'Green' line:
ELSEIF ISNULL([Realizado]) then 'Black'
You'll need to assign colors, but it will work.
Thanks for your answer. It is an alternative, but what I intended to do was to change the color of the whole line text when "Realizado" is null, and still mantein the sign rules.
Do you believe it is possible?