it depends a lot on brake pads. i have different type of pads on front and rear and you can see the difference. i'll upgrade the rear ones in the near future, so the rim stays clean naturally.
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I always use baby wipes though it is a fairly futile task.One ride later they discolour again.
The missus cleans her bike with Muc-Off and her alloy pannier rack has changed colour from black to a purply colour. It could just be wear and tear I suppose, they say it doesn't affect anodising but it definitely won't attack your tyres like white spirit wrecked mine. If the rims are fading I suppose you could say you're going to have to replace them through wear sooner or later any way. I'd have thought anything with a strong cleaning action won't be without its side effects.