Google Sheets - How to format cells through code in Google Sheets. Option 1: Automatically when the spreadsheet opens.

set_color_on_open.gs

// 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");
}

// Will run when user opens/refreshes spreadsheet.
function onOpen() {
  setBackgroundOfScriptDeclaredCell();
}