You are looking for fleas in the straw.
You are looking for something impossible to find. Akin to A needle in a haystack.
No matter how much you wash a black person, you are wasting your soap.
This proverb is mainly used to express the futility of trying to change the mentality of a headstrong person ~ the color of a black person cannot be washed off~ and it does have racist connotations. In modern Greek, the word ___¹__ ~ arapis — black man~ is a lay ~ and at times borderline pejorative~ term — originally probably derived from the word ______ ~ Aravas — Arab~ .
At the deaf man’s door, knock as much as you like.
Some people ignore any advice or guidance that may be provided to them. In more modern Greek, there is a funny spoof of this proverb : ____ ______ ___ ¹____, _¹__ _¹’__ ¹_______ ~ When at a deaf man’s door, get in through the window~ .