Description of Problem: If you set your gnome-panel to have a solid color, the clock inherits this background color for it's background, however, the text of the clock remains black and there is no way to configure it. This becomes very annoying when the color of the panel is set to something dark and the clock becomes unreadable, and mostly indistinguishable except for the white/grey box outline.
Personally I don't think there's a real fix for this (other than using a dark theme with white text to begin with), which is one reason colored panels are a somewhat "toy" feature. You can't make one of the 15 or so GTK colors configurable for something without making them all, which would just be nuts. But probably worth filing on bugzilla.gnome.org to see if someone has a clever solution.
Upstream as http://bugzilla.gnome.org/show_bug.cgi?id=103288