Perhaps this isn't entirely accurate, but there was a time before "zero" as a concept was invented. You don't need it to count. You don't need it to do arithmetic on an abacus. Saying "there are 0 apples" I guess didn't compute. Instead it was "I have no apples". So zero really incorporated the absence of something into the number system and that wasn't always the case.

I wonder if blue was like that for ancient people. It's not that they couldn't see the blue things but that blue, given that it was the color of the sky and arguably the sea, wasn't really a colour at all. It was the absence of colour.

Essentially blue was a baseline and wasn't thought of as a colour at all. Or at least that's the way I've always thought about it.