code

function onOpen() {
  var ui = DocumentApp.getUi();
  ui.createMenu('Replace')
    .addItem('Character conversion (orgcolor→newcolor)', 'replaceTextColor')
    .addToUi();
}

function replaceTextColor() {
  const org_color = "#ffffff"; // enter old color in hex code
  const replace_color = "#808080"; // enter new color in hex code 
  let app = DocumentApp.getUi();
  let body = DocumentApp.getActiveDocument().getBody();
  let obj = body.editAsText();
  const text = body.getText();

  for (let i = 0; i < text.length; i++) {
    if (obj.getForegroundColor(i) == org_color) {
      obj.setForegroundColor(i, i + 1, replace_color);
      Logger.log("Change color: " + i);
    }
  }
}