>categorizing people by skin colour is ridiculous for almost all purposes
Sure. But race is most often about genetic (or ethno-linguistic) heritage, not skin color. A person from Japan might have the same skin color as someone from Greece.
Only three large racial (genetic) groups in Sub-Saharan Africa.
>Which one exactly is the genetically stupid one?
You're trying to turn this into a normative claim, which it is not. And you started with the assumption that people are just saying this stuff for no reason at all.
Sure. But race is most often about genetic (or ethno-linguistic) heritage, not skin color. A person from Japan might have the same skin color as someone from Greece.