Generally in Germany, the ground is neutral apart from the ultras section and also for big games like Der Klassiker and local derbies etc
i found this last time we played Dortmund
Please note:
Please be aware that in the entire southern part of the stadium (SW, South, SE) access is not permitted to those wearing opposition colours.
In the away team stands - that is, in those parts of the stadium for which tickets were purchased via the away team (typically stands 60,61,08 – as well as in stands 70,75, 55-59) access is not permitted to those wearing BVB colours.
Should the away team not take up its entire ticket allocation, tickets for the unneeded stands will be sold to neutrals and BVB fans, and will only be available from Borussia Dortmund, as well as from BVB advance sales points and here on BVB.de. Entry wearing BVB colours is then allowed, and wished-for.