It is extremely frustrating to me that people use the insult 'carrot top' to describe people with red hair because carrot tops are green thank you for listening and another thing red hair is rarely actually red also red onions are clearly purple who is naming all this stuff.....
The hair thing comes from the time when English didn't have a word for orange. That word came with the fruit.
Orange things were previously just called red - see also Robin Redbreast, when a robin's breast is clearly also orange.
The onions, I can't help with.
Genuinely interesting!