You need to add some settings to your appsettings.json file for the RichTextEditor, for example

"RichTextEditor": {
  "Commands": [
     ...
    {
      "Alias": "forecolor",
      "Name": "Change Color",
      "mode": "Selection"
    }
  ],
  "Plugins": [
    "textcolor",
    "colorpicker"
  ],
        "CustomConfig": {
          "textcolor_map": "[ \u0022000000\u0022, \u0022Black\u0022, \u0022a4b69c\u0022, \u0022Green\u0022 ]",

        }
      }