Yellow. Until 3M realized how much money there was in it, Post-Its were always yellow. Not only do they stick to things, but Post-its are real bright to grab your attention. Which is what yellow is pretty good at. Some fire engines are yellow, signs for Slow Children Playing, less interesting yield signs. And gold, the old money standard. Gold gets people's attention. And highlighters. Which most of the time smear the ink, and then the ink gets on the highlighter itself so the next time you use it it smears regardless of the ink on the paper. Then they came out with other colored highlighters, orange, green, blue. But there is still that problem of smearing.