UV − V vs. B − V diagrams of our volume-limited Hipparcos sample. Red points are the most active stars, with log  R′ _HK > −4.5. Green points have −5.0 < log  R′ _HK < −4.5, while blue points have log  R′ _HK < −5.0. The ellipses in the upper left corner of either panel show the median errors in color. To illustrate the dependence on activity, we plot Equations (2) and (4) at the median log  R′ _HK of each bin.