Sets the cell background color to red-yellow-green based on its value. Used on this Google Spreadsheet: https://docs.google.com/spreadsheet/ccc?key=0AvfBqhRA8IzYdEJfNl83R2w0OGRVZUNFaWxlRFlEVGc

cell-color-based-on-value.js

// cell-color-based-on-value.gs
//
// For more information on using the Spreadsheet API, see
// https://developers.google.com/apps-script/service_spreadsheet

// Returns a linear interpolation of y1,y2 based on x value between x1,x2.
function linearInterp(x, x1, x2, y1, y2) {
  if (x <= x1) return y1;
  if (x >= x2) return y2;
  return y1 + (y2 - y1) * (x - x1) / (x2 - x1);
}

// http://stackoverflow.com/a/5624139/
function rgbToHex(r, g, b) {
  return "#" + ((1 << 24) + (r << 16) + (g << 8) + b).toString(16).slice(1);
}

function calculateColor(value) {
  // Too bad Google Spreadsheets do not support HSL notation (as of 2013-10-24).
  // return "hsl(" + linearInterp(value, 0.00, 1.00, 120, 0) + ", 75%, 50%)";

  var n = linearInterp(value, 0.00, 1.00, 0.0, 2.0);
  // background : -webkit-linear-gradient(right, rgb(255, 64, 64) 0%, rgb(255, 255, 64) 50%, rgb(64, 255, 64) 100%);
  var r = linearInterp(n, 0, 1, 64, 255);
  var g = linearInterp(n, 1, 2, 255, 64);
  var b = 64;

  return rgbToHex(r,g,b);
}

// cell is a Range object.
function recolorSingleCell(cell) {
  var value = parseFloat(cell.getValue());
  if (isNaN(value)) return;
  cell.setBackground(calculateColor(value));
}

function recolorTheCells() {
  // Hard-coding the first sheet.
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheets()[0];
  var range = sheet.getRange("G:G");
  var numRows = range.getNumRows();
  var numCols = range.getNumColumns();

  for (var i = 0; i < numRows; i++) {
    for (var j = 0; j < numCols; j++) {
      recolorSingleCell(range.getCell(i + 1, j + 1));
    }
  }
};

function onOpen() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet();
  var entries = [{
    name : "Recolor the cells!",
    functionName : "recolorTheCells"
  }];
  sheet.addMenu("SCRIPT", entries);
};