How change the color of rulers in Visual Studio Code

From the February 2020 v1.43 release, you can set per-ruler colors. Use like this:

"editor.rulers": [
  {
    "column": 80,
    "color": "#ff00ff"
  },
  100,  // a ruler with the default or editorRuler.foreground color at column 100
  {
    "column": 120,
    "color": "#ff0000"
  },
],

See the release notes here: https://github.com/microsoft/vscode-docs/blob/vnext/release-notes/v1_43.md#multiple-rulers-with-different-colors