This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 赮 is uncertain. It combines 赤 (red) with 叚, but the original semantic and phonetic roles are unclear.
The left side 赤 suggests red, and the right side 叚 is obscure. Imagine a rare, reddish artifact whose purpose is unknown.
For カ, picture a car painted in an unusual shade of red: car -> カ, and the red 赤 side hints at the color.