Google-sheets – How to color an adjacent cell based on a 6-digit hex number without adding the “#”

I've just had a go at this, and here's a more flexible function. The previous example couldn't properly handle ranges of selected cells, this one can (but you could also add some better error checking for empty ranges!) Could easily be customized to have other ranges of brightness, hues, auto-calculation of max value, etc.:

/**
 * Adds a custom menu to the active spreadsheet, containing a single menu item
 * for invoking the readRows() function specified above.
 * The onOpen() function, when defined, is automatically invoked whenever the
 * spreadsheet is opened.
 * For more information on using the Spreadsheet API, see
 * https://developers.google.com/apps-script/service_spreadsheet
 */

function onOpen() {
  var spreadsheet = SpreadsheetApp.getActiveSpreadsheet();

  var menu = [({name: "colorize", functionName: "colorize"})];
  spreadsheet.addMenu("HSV Colors", menu);
}

/*
 * Change saturation of cell background colors based on their values
 */

function colorize() {

  // Prompt the user for a max value number.
  var numberRange = Browser.inputBox('Colorize Range',
      'Please enter the maximum number of your range' +
      ' (for example, "2"):',
      Browser.Buttons.OK_CANCEL);
  if (numberRange == 'cancel') {
    return;
  }

  // Prompt the user for a hue number.
  var hue = Browser.inputBox('Colorize Range',
      'Please enter the hue (0-359)' +
      ' (for example, "128"):',
      Browser.Buttons.OK_CANCEL);
  if (hue == 'cancel') {
    return;
  }


  var range = SpreadsheetApp.getActiveRange();
  Logger.log('range:' + range);

  var values = range.getValues(); // [][]

  Logger.log('values:' + values);

  var backgrounds = [];

  for (var row = 0; row < range.getNumRows(); row++) 
  {
    var rowBackgrounds = [];
    for (var column = 0; column < range.getNumColumns(); column++) 
    {
      var val = Number(values[row][column]);
      if (isNaN(val)) 
      {
        val = 0;
      }
      var newColor = HSVtoHEX(hue,255*val/numberRange,200);
      rowBackgrounds.push(newColor);
    }
    backgrounds.push(rowBackgrounds);
  }
  range.setBackgrounds(backgrounds);
}


// http://stackoverflow.com/a/17243070/1536038
function HSVtoHEX(h, s, v) {

  Logger.log('h:'+h+' s:'+s + ' v:' + v);

    var r, g, b, i, f, p, q, t;

    // turn variables into degrees and percentages
    h=h/360, s=s/255, v=v/255;

    if (h && s === undefined && v === undefined) {
        s = h.s, v = h.v, h = h.h;
    }
    i = Math.floor(h * 6);
    f = h * 6 - i;
    p = v * (1 - s);
    q = v * (1 - f * s);
    t = v * (1 - (1 - f) * s);
    switch (i % 6) {
        case 0: r = v, g = t, b = p; break;
        case 1: r = q, g = v, b = p; break;
        case 2: r = p, g = v, b = t; break;
        case 3: r = p, g = q, b = v; break;
        case 4: r = t, g = p, b = v; break;
        case 5: r = v, g = p, b = q; break;
    }

    //http://stackoverflow.com/a/5624139/1536038
  var result = "#" + ((1 << 24) + (Math.floor(r * 255) << 16) + 
      (Math.floor(g * 255) << 8) + Math.floor(b * 255))
      .toString(16).slice(1);
  Logger.log(result);
    return result;
}

You need =E$1 there. (Or, =$E$1 if the formatted range spans multiple columns.)

Putting E1 would compare cells to the string "E1", which is not what you want.

Putting =E1 compares the first cell in your column range to E1, the second to E2, the third to E3, etc -- because the reference is relative. The dollar sign makes it an absolute reference.