How to reference a Plot or a Line

[Taddypole]

In the attached code, I'm trying to color the Zeroline based on the value of "Diff". On lines 90 and 96 of the code, the PlotColors are referencing Index 4. Index 4 is an "Add" statement to add a line in the "Initialize" section. Are Add lines and Add Plots different from an Indexing point of view?

I've been looking at this for a long time and can't figure it out. When Diff is positive I want to color the line Green, when the Diff is negative, I want to color the line Red.

I'd appreciate it if someone can point me in the right direction.

Thanks...
Taddypole...