Add color component to the Style Guide

Context: whatwg/html#11035.

Author: annevk

whatwg.org/style-guide

@@ -43,6 +43,7 @@
  <li>canceling</li>
  <li>cipher</li>
  <li>clonable</li>
+ <li>color/alpha component (not channel)</li>
  <li>color space</li>
  <li>email</li>
  <li>filename</li>