Add GD info to Color Picker FAQ

I've added the following comment to the Color Picker FAQ (http://drupal.org/node/109457) re: needing GD Library. I had this happen on one of my test servers so thought it should be added to the list.

You should also check your status report at Administer » Logs » Status Report (admin/logs/status) to see if GD Library is found/working on your server. This is needed to actually create the images. If it is not, you need to install it or get your host to do so for you.