Javascript - Generate random dark color

As you know RGB at 0,0,0 is black the darkest and it goes toward getting light until (255,255,255) so you can stop it to go above 100, to get only dark colors or say 9 in hex:

Here is jsFiddle

function getDarkColor() {
    var color = '#';
    for (var i = 0; i < 6; i++) {
        color += Math.floor(Math.random() * 10);
    }
    return color;
}

You could use a custom function that takes a hex and darkens it by the percent lum. You can modify it to return whatever you want back

function ColorLuminance(hex, lum) {
  // validate hex string
  hex = String(hex).replace(/[^0-9a-f]/gi, '');
  if (hex.length < 6) {
    hex = hex[0]+hex[0]+hex[1]+hex[1]+hex[2]+hex[2];
  }
  lum = lum || 0;

  // convert to decimal and change luminosity
  var rgb = "#", c, i;
  for (i = 0; i < 3; i++) {
    c = parseInt(hex.substr(i*2,2), 16);
    c = Math.round(Math.min(Math.max(0, c + (c * lum)), 255)).toString(16);
    rgb += ("00"+c).substr(c.length);
  }

  return rgb;
}

You could also just use hsl (Hugh, Saturation, Luminosity or Lightness). The hsl link actually goes through the above code.

Take any random digit from 0-5 as the first digit of your color and then choose the rest of the five digits using your above code.

JS Fiddle: http://jsfiddle.net/xP5v8/

var color,
       letters = '0123456789ABCDEF'.split('')
function AddDigitToColor(limit)
{
    color += letters[Math.round(Math.random() * limit )]
}
function GetRandomColor() {
    color = '#'
    AddDigitToColor(5)
    for (var i = 0; i < 5; i++) {
        AddDigitToColor(15)
    }
    return color
}