Nearly all vertebrates are outwardly symmetrical in colour. The fin whale is not. Its right lower lip, and often the right side of the tongue and mouth lining, is bright white, while the left is uniformly dark. The front third of the baleen on the right is pale creamy yellow; the corresponding plates on the left are dark.
The pattern lines up with behaviour. Fin whales frequently roll onto their right side when lunging into a school of fish, putting the pale flank uppermost and the dark flank down. The usual explanation is that the flash of white herds prey, or that the countershading is optimised for that rolled posture. A related pigmentation chevron sweeps back from the right side of the head.
The explanation is plausible rather than proven. Right-side rolls are well documented, but no experiment has shown that the white lip actually changes prey behaviour, and some individuals show reversed or partial patterns.