there are no bulbs in the set. It has three tubes, one for each color. You can try cleaning the surface of each lens / tube VERY carefully. If the tv hasn't been serviced (ever), eventually the surfaces do also get coated with "grime". The mirror can also be cleaned but is extremely "tender" and can be readily scratched so extreme care is needed there also.
If you look inside the set when it is operating and look at the lenses, you will see any "growth" in the fluid (which is between the lens and crt tube. It is that "stuff" that is blocking the light and likely causing the dark picture.