Blind and colorblind people still know what color the sky is. It's impossible to live long enough to register on a website and have never heard that.
OP edited their comment. it was originally "What color is our logo?"
Ah, sorry about that then.
Our logo is black actually.
You just need to have heard the most famous song from The Mamas & The Papas.
Hopefully members on HN are smart enough to generalize my example to something that may be better or more suitable for their own website, and not just lazily copy-paste my example of a generic question. If you do, I'm not sure you pass the human test.
Too late. My CAPTCHA now reads "What colour is vortico's logo". It's pretty effective. Not one bot signup.
They know it culturally.
grey
UK grey or Seattle grey?
UK is grey, Seatle is gray
Television-tuned-to-a-dead-channel grey?
Usually black, actually; sometimes with white dots.