Many years ago, when I worked at the British Standards Institution as secretary to the colour-fastness committees, the Post Office telephoned in a flap. The hand-written entries on tax discs had faded. It seems someone had changed the light-fast ink used previously for cheaper ordinary ink. I would suggest your correspondent who suffered faded ink on his disabled badge complains to the issuing authority as I think it is reasonable to expect that the entries on disabled badges should remain legible until the expiry date. The light fastness of the ink should be as good as curtains, which, if I remember correctly, is 8.