Green color on old CRTs is dying.

I have two old CRT monitors where the green occasionaly (usually after the CRT warms up) cuts out turning the screen purple. Both monitors are the DSUB to BNC style cables. Both are several years old, but they're huge, 19" & 21" and still good.

Both monitors have the same problem. Swapping the cables doesn't help. I've used them on multiple computers.

Is there anything I can do?

Dry/fractured solder joint on the green cathode gun. If you're lucky, the electronics controlling it will be accessible once the case is removed, and you can just remelt the joint with a soldering iron.