urxvt uses pink instead of solarized until I run xrdb ~/.Xresources

Ok, I figured it out: You have to remove the -noccp option in line 42 of /usr/sbin/lightdm-session, which prevents xrdb to use the preprocessor I needed.

If ~/.Xresources are not processed with cpp when you login, you may want to save your original source as another name and rebuild ~/.Xresources every time you make changes.

I just wrote a tool to automate this process: https://github.com/dolmen/Xresources.edit/