Here is plausible solution.
Change the calculation
% diff YOY KPI applications
IF [% diff YOY applications]> 0 THEN "Up"
ELSEIF [% diff YOY applications]< 0 THEN "Down"
ELSEIF [% diff YOY applications]= 0 THEN Null
But I suspect that you want to show that it is level.
In the color legend, double click on the off-white color next to Null (in the Select Data Item box). A color dialog box will appear with white being an option in the bottom right corner. Then, to hide the nulls even further, I noticed that changing the circle shape to the right pointing arrow from the Thin Arrows shape palette, hid them quite well.
Hope this helps!
Thanks so much, Tracy!
It was the double-clicking on the legend that I hadn't figured out
All the best