A Google Sheets script which adds color preview to cells. When you edit a cell containing a valid CSS hexadecimal color code (like #000 or #000000), the background color is changed to that color and the font color is changed to the inverse color for readability.

google-sheets-color-preview.js

/*

This script is meant to be used with a Google Sheets spreadsheet. When you edit a cell containing a
valid CSS hexadecimal color code (like #000 or #000000), the background color will change to that
color and the font color will be changed to the inverse color for readability.

To use this script in a Google Sheets spreadsheet:
1. go to Tools » Script Editor;
2. replace everyting in the text editor with this code;
3. click File » Save;
4. set the title to "Set color preview on edit".

This will apply to cells when they're edited. If you already have values, you can force them to
update by cutting & pasting them over themselves.

Includes improvements from...
- wjmazza (tweaked regex);
- XCompWiz (optimised when editing larger ranges).
*/

/*********
** Properties
*********/
/**
 * A regex pattern matching a valid CSS hex color code.
 */
var colorPattern = /^#([0-9a-f]{3})([0-9a-f]{3})?$/i;


/*********
** Event handlers
*********/
/**
 * Sets the foreground or background color of a cell based on its value.
 * This assumes a valid CSS hexadecimal color code like #FFF or #FFFFFF.
 */
function onEdit(e) {
  // iterate over cell range  
  var range = e.range;
  var data = range.getValues(); // read all the data up-front (much faster than reading each cell individually)
  var rowCount = range.getNumRows();
  var colCount = range.getNumColumns();
  for(var r = 0; r < rowCount; r++) {
    for(var c = 0; c < colCount; c++) {
      var cell = range.getCell(r + 1, c + 1);
      var value = data[r][c];
      
      if(isValidHex(value)) {
        cell.setBackground(value);
        cell.setFontColor(getInverseHex(value));
      }
      else {
        cell.setBackground(null);
        cell.setFontColor(null);
      }
    }
  }
};


/*********
** Helpers
*********/
/**
 * Get whether a value is a valid hex color code.
 */
function isValidHex(hex) {
  return colorPattern.test(hex);
};

/**
 * Get a hex color code that is the inverse of the provided code.
 * Derived from stackoverflow.com/questions/9600295 with added
 * support for CSS shorthand hex notation.
 */
function getInverseHex(hex) {
  // expand shorthand color
  hex = hex.replace(/^#(.)(.)(.)$/, '#$1$1$2$2$3$3');
  
  // convert hex to decimal value
  var inverse = parseInt(hex.substring(1), 16);
  
  // invert color
  inverse = 0xFFFFFF ^ inverse;
  
  // convert back to hex notation
  return '#' + ('000000' + inverse.toString(16)).slice(-6);
};

For some reason I can't get this working. Everytime it runs I see this error in the console:
Google Apps Script: TypeError: Cannot read property 'setBackground' of undefined