Adding silhouetteColor to the model created by Cesium.ModelInstanceCollection is invalid. Is there a solution?
When doing collection._model, you are trying to access a private member of collection (hence the underscore). This causes collection._model to return undefined, and you can’t access silhouetteColor as a result.
This is a sandcastle example of using entity collection instead, which allows you to access the model and changing its silhouette color: Sandcastle example.