URXVT - Xresources doesnt get parsed on boot
When the Xresources file contains variables and the packaged xrdb doesnt contain the cpp parser, a manual parsing might be required.
.Xresources file will be saved as
.Xresources.dev. A new parsed .Xresources file can be generated by the following command.
$ cpp -P .Xresources.dev .Xresources
.Xresources should be applied correctly.
Source: Launchpad Bug report