I am not sure what you mean by reflective mirror? I think what you mean something to reflect photons upwards, but I could be wrong. There are tutorials on youtube that can show you how to do this.
Use FILT with .tmp = 1 which will allow only those photons to pass through it which are of same colour as FILT but will block all other photons.
For more information about tmp modes refer to this page https://powdertoy.co.uk/Wiki/W/Element:FILT.html