Google Sheets - How to format cells through code in Google Sheets. Option 2: When user initiates your script

set_color_when_user_clicks_menu.gs

// Add custom menu to spreadsheet
function onOpen() {
  var ui = SpreadsheetApp.getUi();
  ui.createMenu('Run custom script')
      .addItem('Set background of a script-declared cell (A1)', 'setBackgroundOfScriptDeclaredCell')
      .addToUi();
}

// This does not set the color of a user-selected cell like using getActiveCell does, but it necessarily relies on getActiveSheet(), since ranges are defined as referencing cells within sheets.
function setBackgroundOfScriptDeclaredCell() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var range = sheet.getRange("A1");
  range.setBackground("red");
}