bay
騢 refers specifically to a bay horse, a horse with a reddish-brown body and black mane, tail, and lower legs.
騢 combines 馬 (horse) with 叚, which likely contributes the sound and may relate to the reddish color of a bay horse. The exact historical development of 叚 in this context is uncertain.
The left side is a horse 馬, and the right side 叚 looks like a stylized mane and tail. Picture a horse with a dark mane and tail against a reddish body: that's a bay horse.
For カ, imagine a bay horse galloping past a car: car -> カ, and the horse's reddish coat flashes by.