The color blue associated with cold or freezing temperatures. The colour blue is associated with many things: virginity or purity in general, masculinity (and oddly enough, femininity as well), serenity, the sky, water, sadness, heroism, Another thing it's associated with is cold temperatures, and this is what this trope is about. If a water tap has blue on it, it's pretty much guaranteed to produce cold water unless there is a flaw in it somehow. Anybody who feels cold.