If it's a girl, don't forget to paint the room pink and get pink curtains. Pink is an inherently girly color that makes us think of flowers and sweet smells and being delicate, while blue is, uh, football, Chevy trucks ... Smurfs ... that topless lizard chick from Avatar ...
But at One Time ...
If it's starting to seem pretty arbitrary, that's because it totally is. Up until the start of World War I, people didn't care what color their kids' diapers were, because it was the freaking 19th century. What color the fabric is under the baby poop is the last thing on your mind when you have to deal with insanely high infant mortality rates, the Civil War, cholera and roving packs of baby-eating wolves (look, the old days were hard, OK?).
"Don't worry, Junior, dogs are your friends!"
Luckily, all our gender issues were heartily resolved by the 1910s, when it was decided that we'd assign colors to each "team": blue was for girls and pink was for boys. No, that's not a typo: A 1918 editorial from Earnshaw's Infants' Department stated that pink was "a more decided and stronger color ... more suitable for the boy; while blue, which is more delicate and dainty, is prettier for the girl." It makes sense: Pink is the color of a nice, raw, manly steak, or the blood of your enemies splattered on a white uniform.
Which is why Matt Stone appears to be far manlier than Trey Parker.