A university has had presidents, and their portraits line a hallway of the student center. Every year, on April Fool’s Day, students from the senior class get together to pull a prank: they paint over the portraits in an elaborate way. At the end of the prank, some portraits are yellow and some are blue — but how can we tell which color a given portrait will be? It turns out we can find the answer without walking down the hallway and checking.
To get started, keep reading, or skip to the challenge to answer the question.