Starting a program from a command line versus clicking an icon.

[walt]

I've written a little C program named rlc which generates
random color strings like 'rgb:a5/b3/f1' which I can use
like this to start xterm:
xterm -bg `rlc` (gives a random-light-colored background)

This works great when I type the command from an xterm
window -- but it fails when I click on a gnome or xfce
icon containing the same command.
Any idea why those two methods of starting a program
don't produce the same result?
Thanks.