/etc/X11/app-defaults/Clock-color is in x11-apps 7.6+5ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | ! $Xorg: Clock-col.ad,v 1.3 2000/08/17 19:53:59 cpqbld Exp $
Clock*Background: grey
Clock*BorderColor: light blue
Clock*hour: yellow
Clock*jewel: yellow
Clock*minute: yellow
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