Hello L.
You can use, but for all diamond:
skinparam activityDiamondBackgroundColor purple
With that:
@startuml
skinparam activityDiamondBackgroundColor purple
repeat
:read data;
:generate diagrams;
repeat while (<color white>**more data?**</color>) is (yes) -[#blue]->
@enduml
We observe:
If that can help,
Regards,
Th.