Hi Jerome,
Thanks for contacting Syncfusion support.
Requirement: Need to change the Connector color while selecting the Node.
We can achieve your requirement through ItemSelected event in our SfDiagram. We have prepared a simple sample for your requirement. Please find the sample link below. In the sample, we have changed the color of the incoming connector of each node through ItemSelected event and when we unselect the item the change the color of the connector to its default in the ItemUnselected event.
For more example like this please follow our FlowExecution sample. Please find the link for DiagramDemos à FlowExecution sample below.
Regards,
Karkuvel Rajan S