black horse with white mane
雒 centers on a specific horse color: a black horse with a white mane. The meaning is narrow and concrete, with no clear broader conceptual extension.
雒 combines 各 and 隹. The exact historical development is uncertain, but it likely originally referred to a type of bird or animal name, later specialized to the horse color meaning.
The left side 各 (each) and right side 隹 (bird) can be remembered as a bird with distinctive coloring: imagine a black bird with a white crest, like the black horse with a white mane.
For ラク, picture a black horse with a white mane relaxing in a field: relax -> ラク, and the horse's distinctive coloring makes it easy to spot.