When the case demands a high strength zirconia material and the restoration is in the secondary smile zone (shadows of the smile), then I am thinking pre-sintering infiltration coloring.  As of late, my preference is the ZirCAD LT Colouring Liquids.  This video will show the steps and liquid color application technique I use for a premolar bridge.  My primary coloring objectives for green zirconia is creating the saturation multi-effect and lowering the value in the cusp and occlusal zones.  It rather simple to learn and master.  

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