The G2 setting on Most CRT's in home cinema is a once in a few years setting and does not require constant tweeking.
Each projector has its own way of adjusting and the below is specific to the Barco Graphics and Data 1209 which did not have voltage indicating LEDs to help identify the correct setting.
Feed a colour bar signal into the composite video port.
Go to the random access section of the barco menu and select colour balance. go to the black balance (could be called cut off) and make sure the red,blue and if the option is give that green is set to 50%.
Set the contast and the colour 0%. Set Brightness to 50%
Short circuit the two small pins that you can see on the black end stage of the tube, next to the phono plug that inputs the video signal. (this will switch off the automatic black level ABL)
Black the room out completely. cover the tubes that you are not adjusting and turn the respective G2 pot so that you can just see an image apear on screen. (repeat for the other colours)
The G2 pots are the coloured chappies on the board on the far right of the projector when table mounted and viewed from behing.
A few warnings here. There are some leathal voltages on that board. the heat sinks are live to the mains and very close to those pots are some components that if you short them out will blow the projector big time. I suggest you only use a plastic adjuster and if you are not sure then leave it alone.
set your brightness, colour and contast back to normal and adjust the colour balance.