On Sun, Dec 13, 2015 at 11:51 AM Eli Zaretskii <eliz@gnu.org> wrote:
>
> > I assume you meant `tty-color-mode-alist'.
>
> No, I meant the value returned by tty-color-alist, which is stored in
> the variable tty-defined-color-alist.

Aha!  As Emily Litella says, "That's quite different" :-)

So I added some "message" calls, as described by the attached patch.
Here's what I see in *Messages* after simply running `emacs -Q`:

    terminal-init-xterm doin’ its thang.  (tty-color-alist) returns a
list of length 8.
    terminal-init-xterm is done.  (tty-color-alist) now returns a list
of length 256.
    For information about GNU Emacs and the GNU system, type C-h C-a.

And here's what I see after I create the new frame:

    You can run the command ‘server-start’ with M-x ser-s RET
    terminal-init-xterm doin’ its thang.  (tty-color-alist) returns a
list of length 256.
    terminal-init-xterm is done.  (tty-color-alist) now returns a list
of length 8.
    When done with this frame, type C-x 5 0

So clearly: xterm.el initialization _is_ getting called for the client
frame, and it _does_ modify tty-color-alist.

I've put the actual before-and-after values returned from
`(tty-color-alist)' into a second attachment, since the "before" value
is quite long.