Well, dust has nothing to do with the problem. This sounds like a thermal problem with the chrominance circuit in the television. It would be repairable, but you should probably consider replacing the television. It is a much older model and, unless it was a very good one to begin with, will have an inferior picture compared to many modern televisions. The oeprating life of a CRT television is about 15 to 20 years, so yours is getting up there. Colour television signals, in the NTSC system used in the US, Canada, and Japan, are a kludge on top of the older B&W standard. B&W broadcasts contained liminance information, which described how bright the image should be at any given instant. As the electron beam drew the image, scanning across the screen, the luminance was varied. When colour television was developed, they did not want ot lose compatibility with the existing NTSC B&W sets, so a separate chrominance signal was squeezed into the available bandwidth for exach channel. This signal described the colour of the beam as it scanned back and forth on the screen. Luminance (brightness or how white) plus chrominance (colour or which colour in the rainbox to reproduce) produces a colour image.