I is for...

Iconic
If someone describes a colour as Post Box Red, Cadburys Purple or British Racing Green you know what they are referring to. No idea why post boxes in the UK are red and different colours in other countries - I noticed they're blue in Sweden when we were over there last year. 
I've only used this post box once last year - to post Christmas Cards - another one of those digital/analogue things - maybe we should start writing letters more?