I've just checked my denim mabel, bluecat. The handles are a bit faded, but otherwise it's ok, I think yours looks ok too - I can't see the yellow but maybe its the pics...?
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I agree the pics don't seem to pick up the yellowish tinge, its more obvious in some lights than others. However, I wouldn't really expect a £600 bag to fade to that extent after 6 months of very gentle use.