Replace undefined G4_Al colour with 'grey' to suppress Geant4 warnings

[zdemirog]

This PR suppresses Geant4 visualization warnings. The undefined G4_Al colour is replaced with the standard grey colour in the visualization settings to avoid these warnings.

-------- WWWW ------- G4Exception-START -------- WWWW -------
*** G4Exception : greps0003
      issued by : G4Colour::GetColour
Colour "G4_Al" not found. No action taken.
*** This is just a warning message. ***
-------- WWWW -------- G4Exception-END --------- WWWW -------


-------- WWWW ------- G4Exception-START -------- WWWW -------
*** G4Exception : greps0003
      issued by : G4Colour::GetColour
Colour "G4_Al" not found. No action taken.
*** This is just a warning message. ***
-------- WWWW -------- G4Exception-END --------- WWWW -------


-------- WWWW ------- G4Exception-START -------- WWWW -------
*** G4Exception : greps0003
      issued by : G4Colour::GetColour
Colour "G4_Al" not found. No action taken.
*** This is just a warning message. ***
-------- WWWW -------- G4Exception-END --------- WWWW -------