The advantage Orichalcum has over Mythril is its unique color. Mythril Ore does not stand out as much and bears resemblance to other ores and blocks.
In real life, orichalcum is a legendary metal described by the Athenian philosopher Plato in his work Critias to have been mined in the lost city of Atlantis. It is suspected to have been a sort of alloy involving copper.
The placed sprite looks like waves of a thick ooze flowing down a vertical surface. This would match the theme of orichalcum’s “brick” variant being Bubblegum Block.
While the in-game color is magenta, orichalcum is speculated to be a more reddish hue.