Tickled Pink

Builds on the much older use of "tickled" to mean delighted, which Shakespeare himself used in Coriolanus ("tickled with good success"). The "pink" half is newer, dating to the late 19th or early 20th century, and the leading theory holds that it pictures someone's face flushing a happy shade of pink from laughing or blushing with pleasure, similar to the older phrase "in the pink," meaning in good health. It's a well-supported, plausible theory rather than a wild guess, but the exact link between "pink" and happiness specifically, rather than just health or good condition generally, isn't fully nailed down by any single documented source.