I know you can run shell scripts from AppleScript, and Applescript seems to be the foundation of Automator (at least you can use applescripts). As such I can think of ways to use curl to grab the text of a webpage, then various other tools to look for the hex for a color name or its name, then pass a return value back up to AppleScript, then back to Automator. I'd do it all in shell, or a CLI app, but if other parts of an Automator workflow already exist, i guess leveraging that might make sense.
If Automator is the only tool one knows, this might be very difficult, though. It may be possible via AppleScript only to ask safari for a page's source, and do it all via AS, but I wouldn't know how to begin down that route.
-Lee