Assuming the user picture controls work properly and the image is not
gray when the color is turned off:

Could be a worn out picture tube that will no longer properly balance
or a failure in the video circuitry. You really need to have the video
output voltages and drive levels checked to confirm that there is no
failure before cranking on controls.

In short, the drives control the high level on the tube and the
background sets the low level.

If readjusting:

Depending on the specifics of the model the drives probably need preset
to mid control for the blue and green to set the base drive levels the
same as the red.

Then turn all three background controls all the way back to minimum.

Apply a black and white gray scale bar pattern on screen. If the set
has a service switch then it needs to be set to give a single line
across the screen.

Turn up the G2/screen voltage until one of the gun colors is clearly
but slightly visible across the screen. If there is no service switch
then the 10 IRD gray bar should be used for the background adjustment.

If it is red or green then the other colors background, red or green,
needs adjusted to make the line yellow. Then adjust the blue
background to make the line as close to white or light gray as
possible.

Turn off the service switch and look at the lightest color of gray
before the white. Adjust the blue and green drive to make the
highlights look as close to gray and white as possible.

Given the age of the set it is probable that the crt will not track
properly from low light to high light gray scale. Get it as close as
possible.

Otherwise it might be helpful to have a technician do the gray scale
adjustment that has lots of experience. They will be able to generally
get it close by eye and fudge the adjustment a little even on a weak
tube to get a watchable picture.