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Use: skinparam ActivityDiamondBorderColor black
Hello P.,
If it is for Activity Diagram, You can use LineColor and new style feature like:
<style> diamond { LineColor black } </style>
With this result:
[click to see on PlantUML online server]
If that can help,Regards,Th.
Thank you for your time but for me didn't work in that way but you used diamond keyword and I knew what to find in skinparam.
In the end I used: skinparam ActivityDiamondBorderColor black