I've seen Iberia(!) staff refuse to believe that a BA Silver card holder was allowed access, yet an IB Silver wasn't. (And this was post merger).
Tell customers that this is 'unfair' and 'a british attack on Spaniards' is not good customer service.
When the passenger (with my voluntary support) began explaining oneworld levels, they told him he was lying!

Eventually resolved after some feet stamping.
Although as a BA Gold I was told 'we are just letting you in as a favour, the IB Platinum (another person nearby) agrees you are not allowed in.


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The coloured circles don't work in practice, they are not conspicuous enough, and the colour can look a bit different against different designs.
Needs a more significant symbol.

This would also stop people like Darius from MillionMileSecrets doing what they do claim they do on their blog, flashing other cards to fraudulently enter security lines, premium checkin etc.