The ArcGIS Runtime for .NET doesn't support overwriting the SceneLayer symbology in 10.2.6. This is why there is no renderer for the SceneLayer. If you want to change the color of the buildings, it needs to be done while publishing the service. But later this year or early next year, SceneLayer will allow you to publish points features to be consumed in a 3D environment (in addition to the current SceneLayer capabilities that only supports Mesh Data: like buildings...). That will require being able to change the symbology the same way you can using a feature layer. So when the ArcGIS Runtime will support that new type of SceneLayer you'll be able to set a Renderer for it, and you'll be able to change the Color of even Mesh SceneLayers.
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