You can tell *xterm* to do that by adding

        #ifdef        *COLOR*
        *XTerm**termName: *xterm*-*color*
        #endif

to your .Xresources file (or whatever file is being fed to xrdb to set
resources). If you're not using anything, then

        echo '*XTerm**termName: *xterm*-*color*' | xrdb

in .xinitrc - before you start any xterms - will do the job, except
that will also set it in the unlikely event that you are on an X
server that doesn't support *color*.