Generally it's possible to do, occasionally there's even a service option that disables red and blue. A color CRT with only one gun running still doesn't look the same as a monochrome CRT because it has a shadow mask and the phosphor is different. Mono tubes are still out there, I bought a NOS monochrome CRT from Video Display Corp last fall to replace a tired one in my Wyse terminal , the shipping can be a killer on those though.
Save CRTs when you find them though, they're not being made anymore and nobody is rebuilding them either. When they're gone, they're gone.