I like this explanation. It also tells how to set different colors for more than one ruler.

I like this explanation. It also tells how to set different colors for more than one ruler.

Color and multiple vertical rulers are available in VS Code as of the February 2020 edition.

For example:

"editor.rulers": [
  {
    "column": 80,
    "color": "#ff9900"
  },
    100,
  {
   "column": 120,
   "color": "#9f0af5"
  },
],

To target a specific language:

"[ruby]": {
  "editor.rulers": [
    {
      "column": 100,
      "color": "#00ff22"
    }
  ]
}