Well, if you select one node, yEd will show the nodes that are directly connected to the selected node in its "Neighborhood" context view. You can even convert said neighborhood into a new diagram (right-click into the "Neighborhood" context view to open its context menu and select "Convert to Document").
Alternatively, you can use "Tools" -> "Select Elements" with the following settings to extend the selection to all nodes that are directly connected to an initially selected node:
- Uncheck all options on tab "General".
- Check "Use These Criteria" on tab "Nodes".
- Set "Select" to "Neighbors of Selected" and "Maximal Path Distance" to "1" on tab "Nodes".
Additionally, you could use "Select Elements" to invert the selection and turn all selected nodes invisible by changing their "Fill Color" to "No Color".
"Select Elements" also supports selecting all nodes with a specific property value.
Finally, yEd's properties mapper can be used to change e.g. the fill color of nodes depending on specific property values.