coloured
碏 centers on the idea of being coloured, likely referring to a stone or mineral with distinct hues. The meaning is narrow and the kanji is rare, so treat its usage as uncertain.
碏 combines 石 (stone) with 昔, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound related to stone or colour.
A stone 石 that looks like it comes from the past 昔, weathered and coloured by time.
For シャク, imagine a coloured stone that is a shack decoration: shack -> シャク, and the stone adds colour to the shack.