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I can only guess. It sounds like it is gassy, but not up to air. The color is probably due to an oxide layer. The thinner layers produce vivid colors by interference. When it gets thick it just looks black. The part by the filament would get the hottest and have the thickest layer.
Is part of the phosphor on the screen missing or is there a burn in the screen? If the screen had a bad burn from losing the vertical scan it could produce gasses including sulphur.
John
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