That depends on what YOU mean by "standby mode". I don't know about putting your actual computer itself in standby mode, but to make the monitor switch off & it's LED turn orange or red, do the following:
You need to have both a DPMS capable monitor, which most are, & have DPMS enabled in your X config file.
Find the line in your X config file, which is either /etc/X11/xorg.conf or /etc/X11/xfree86.conf, that configures the monitor, mine is:
ModelName "Monitor 1600x1200"
HorizSync 30.0 - 72.0
VertRefresh 47.0 - 110.0
Note the line:
Make sure that line is there.
Then restart X & at a terminal type:
xset dpms force standby
to enter standby mode, or to switch the monitor off completely, type:
xset dpms force off
Just type xset to see any other options.
Hope this helped, let me know how you went.