A piece of yellow paper
I had been doing Tech Support for Hewlett-Packard's DeskJet division for about a month when I had a customer call with a problem I just couldn't solve. She could not print yellow. All the other colors would print fine, which truly baffled（迷惑） me because the only true colors are cyan（蓝绿色）, magenta（洋红）, and yellow.
I had the customer change ink cartridges, delete and reinstall the drivers. Nothing worked. I asked my coworkers for help; they offered no new ideas.
After over two hours of troubleshooting, I was about to tell the customer to send the printer in to us for repair when she asked quietly, "Should I try printing on a piece of white paper instead of yellow paper?"