this isn't entirely correct. if you take the distribution of all extant cat pixels, you will see that pink is very rarely observed (there are no naturally pink cats, but there are certainly pictures of cats dyed pink). Therefore, if you see a pink pixel, you can say with modest certainty that it's not a cat. That's a fairly pedantic and uninteresting point, but the reality is even scalar observations contain information and can be used to classify, if poorly.