This is just kinda obstinate though. Tortoiseshell in the context of cats obviously refers to the fur pattern. To assume otherwise is odd. And just because that’s the kind of tortoiseshell you’re thinking of doesn’t mean the pattern isn’t also named after a tortoiseshell.
I’m not talking about the color variant (which doesn’t look like a toirtoise at all, really). OP’s cat’s belly looks like a literal tortoise shell.
This is just kinda obstinate though. Tortoiseshell in the context of cats obviously refers to the fur pattern. To assume otherwise is odd. And just because that’s the kind of tortoiseshell you’re thinking of doesn’t mean the pattern isn’t also named after a tortoiseshell.