Extensions are normally not allowed to pick colors from local
file:// URL addresses.
If you really need it, you can enable Allow access to file URLs option in Chrome Extensions page.
Because this allows extension to access your local files and thus introducing potentional security problem, this can't be done from extension settings, but must be explicitly allowed through Chrome's settings.
As Eye Dropper can't access all pages but only current one, it might be.
I would recommend against enabling this (at least permanently) because extension can get broader permissions later getting access toall files on your computer. I'm not planning to require more permissions in Eye Dropper currently, but be carefull anyway.
If you are aware of the risks, lets do it:
Allow access to file URLs
You can of course enable/disable it in same manner any time you need.