I successfully formatted a label background color based on a CASE expression (linked to the Border color property):
CASE WHEN "NextStep" = 'closed' THEN color_rgb(0,0,255) ELSE color_rgb(255,0,0) END
However, when creating the composer, this distinction of label background color is not reflected...
The only categories that are listed in the legend is based on the style categories.
What do you suggest to get a clear legend taking into account the various label format based on specific expression?