title: How to check if a string is a valid hex color representation?
body: <p>For example:</p>
  
  <p><code>AA33FF</code> = valid hex color</p>
  
  <p><code>Z34FF9</code> = invalid hex color (has Z in it)</p>
  
  <p><code>AA33FF11</code> = invalid hex color (has extra characters)</p>