If you haven't already, swap Q18 with Q20. If the screen is now red instead of green, one or both of Q18/Q20 is bad, Q19 may also be bad; and also, your tube is (probably) good.
Compare (voltage) the bases of Q18/Q19/Q20 (if it's more convenient, follow the traces and test them at the connector; the schematic doesn't give a board marking for it). If Q18 is much different the others, your tube is (probably) good.
If not, compare R106/R107/R108 (lift a leg); their resistances should match (+/- whatever), likewise R96/R100/R104 (don't need to lift a leg on these). Replace any outliers, then try it again.
If the problem persists, set VR11/VR12/VR13 to match each other (I would meter them to be sure but it's not critical), pull Q18/Q19/Q20, power it up with the transistors not present (you won't see anything on screen; nothing is driving the guns, so this is normal), and compare (voltage) the holes where the emitters would be. If Q18's hole is significantly different from the others, chassis problem. If they're all the same, it's the tube. If you've verified R96/R100/R104, and the COLLECTOR holes are still different, then it DEFINITELY is the tube. FWIW, I'm not 100% sure it's safe to power up the chassis with the CDTs not present, but hey, what have you got to lose?
If you just wanna shotgun it, verify every resistor on the neckboard (except R105) by comparing it against its counterparts on the other channels, replace the caps (skip C49 if you want) and Q18/Q19/Q20, then check the drive signals from the HV board (bases of Q18/Q19/Q20). An outlier there indicates a problem on the HV board that'll throw the next test off. If everything checks out, turn VR12 all the way down (max resistance), VR11 and 13 all the way up (0ohm or close to 0). You should have a purple (red+blue) screen now. If you still have greenscreen, or nothing at all, it's the tube.
As an aside, instead of tossing it, I would troll eBay for cheap rejuvenators. Every now and then one turns up in an estate sale, or a storage locker, and gets sold off by someone that has no idea what it is other than it says "CRT Rejuvenator" on the front of it. I got a Heathkit from an estate seller for $25.