function onOpen() {SpreadsheetApp.getUi().createMenu("Scripts").addItem("Number by Colors", "numberColors").addToUi();}

function numberColors() {
  // ⬇ script 'Settings' below. UPDATE IF RESTRUCTURING OR RECOLORING SHEET! ⬇

  var colorCol = 6,     // 6 is F
      numberCol = 7,    // 7 is G
      firstDataRow = 2, // first row in which to begin color checks (to skip headers)
      palette = [
        "#ffffff",      // white (or your spreadsheet default)
        "#d9ead3",      // light green 3
        "#d9d9d9",      // light gray 1
        "#f4cccc"       // light red 3
      ];

  // ⬆ script 'Settings' above. UPDATE IF RESTRUCTURING OR RECOLORING SHEET! ⬆

  var numbers = [],
      activeSheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet(),
      dataNumRows = activeSheet.getMaxRows()-(firstDataRow-1),
      colorColColors = activeSheet.getRange(2, colorCol, dataNumRows, 1).getBackgrounds();
  for (var rowIndex in colorColColors) {
    numbers[rowIndex] = [];
    var rowColor = colorColColors[rowIndex].toString(),
        colorIndex = palette.indexOf(rowColor);
    if (colorIndex == -1) {
      numbers[rowIndex].push(rowColor); // custom output when cell color is missing from script palette.
    } else if (colorIndex == 0) {
      numbers[rowIndex].push(""); // custom output for first palette color (spreadsheet default). "" means blank.
    } else {
      numbers[rowIndex].push(colorIndex);
    }
  }
  activeSheet.getRange(2, numberCol, dataNumRows, 1).setValues(numbers);
}