To me it's shorthand for "It took years of practice to get good and I can't explain the process I use. The problem appears too complex for a scientific approach to be worth the effort. I certainly can't think of any polynomial-time algorithm and my experience has led me to doubt that one exists."
It's a caution for beginners. The expert is basically saying that getting good at it will take a lot of work and finding a good systematic approach is likely to be a dead end.
They may not know how to deduce what the perfect painting would look like, but that doesn't mean that it's not possible.
One can certainly brute-force the issue by creating every possible painting and comparing them. Whether this can be done efficiently is another matter altogether.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.