Thanks for the questions, the functionality you're referring to is
totally under-documented. Hopefully I can rectify this by writing
detailed instructions on
www.new-tab-jumpstart.com after we resolve
this here.
1) That's CSS acceptable value. So, this can be color name (whatever
Firefox can distinguish like Red, Yellow, Aquamarine etc.) or color
code like #FFFF00 (for yellow for example) or rgb(255, 0, 0) (this is
red for example).
2) After setting path to your image you only need to refresh the page
or open a new tab. However, if it is not working, I'd need to know
which operating system and Firefox version you're using (this was
tested on Windows and Linux, but not on all possible versions) and if
I can't reproduce it I'll need you to send me path to the file as
shown in text box of the extension properties (send it as a personal
message please so it's not publicly visible) since it is possible that
there's a bug in path -> url conversion.
Regards