By default only colors 0-15 are read from X resources by urxvt, this extension loads the rest.
Simply place the xresources-256 file in either ~/.urxvt/ext/ to install it just for your user, or /usr/lib/urxvt/perl/ for a system-wide installation.
To enable this extension just add it to the end of the list of perl extensions in your ~/.Xresources file, for example:
URxvt.perl-ext-common: default,xresources-256