You could try X-referencing the Phase 1 drawing into your Phase 2 file, and through the layer property manager change the x-ref colors to gray? Or set the transparency only those x-ref layers? This should leave your main Phase 1 file the way it is, and only override it in the phase 2 file. If you go with the transparency route, make sure you set the plot option to plot transparency
Edit: Wait, I may have jumped the gun. You're asking for this in Revit?