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If the yellow color is the only one that fades on one side, then you have a problem in the yellow section of the print engine. I would advise you to look in the yellow section, see if there is any spilled toner (of any color whatever) obviously there, and also check the yellow toner level (and if it isn't at least 1/3 full, contemplate replacing the yellow cartridge.)
If the fading is only in one color, also, then the drum wasn't defective, unless it was the drum for just that one color - and some color printers don't have a drum for each color, it varies with the design. Was the drum you replaced 'The' drum? or just the yellow drum? If there is a drum for each color in the model 7535 (I can't recollect offhand, I confess), then the yellow drum only would need replacing, of course.