Agatha Christie - The Harlequin Tea Set

It’s been many years since Mr. Satterthwaite has seen Mr. Harley Quin, so when Satterthwaite, awaiting his broken down car, goes to a tea shop called the Harlequin café, he begins to think of his friend. A self-described snob, Satterthwaite orders coffee and examines the coloured china when a bolt of sunlight comes in and the very same Mr. Quin walks through the door. Enigmatic as ever Mr. Quin and his diligent dog Hermes stay for a Turkish coffee with the excitable Satterthwaite whilst the car is fixed, and Satterthwaite cannot help but bore Mr. Quin with the very long history of the family he is off to visit. Their conversation is interrupted by the abrupt entrance of the member of that very same family intent upon replacing her harlequin cups. Satterthwaite desperately persuades Quin to accompany him, but, all the bereft Satterthwaite is left with is one word, ‘Daltonism.’ What is the importance of Quin turning up at the tea shop on that day and what does that word have to do with anything?