Blue

Blue is Percy Jackson's favorite color, and also a kind of running gag throughtout the series.

History
Gabe Ugliano once told Percy's mother Sally Jackson that there was no such thing as blue food. From that point on, she went out of her way to find blue things. Also, Percy's favorite color is blue.

The Last Olympian
Percy promises his mother that if the Olympians win, he would give his mother some sort of sign, something blue. After Kronos is defeated, he asks Zeus to light up the top of The Empire State Building blue to let his mom know everything is okay.