Why are Post-It notes the colour they are?

How to Make Love: A 1936 Guide to the Art of Wooing
London Underground ghost stations
Christmas travel
How to Make the Perfect Cup of Tea: George Orwell’s 11 Golden Rules
London’s Bill of Mortality (December 1664-December 1665)
Examination of Conscience for Boys and Girls
Tubist Trompe-l'Oeil
Little Blue Book series
Soho 1983: A Girl's Guide To Peep Shows, Topless Bars & Nude Encounters

A map of abandoned railway lines in Britain.
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Source: F. Dans (flickr)