Sorry, I should have linked this yesterday
http://www.subsim.com/radioroom/showthread.php?t=112868
That sort of explains it a bit. In the case of the tut its explaining the "transparency" but I think if you start to look at it you might understand what needs to be done.
Just a small Wiki reference as to what Alpha Channels is
http://en.wikipedia.org/wiki/Alpha_transparency
I would also recomend you to download the better scope Mod from l3th4l
http://www.subsim.com/radioroom/showthread.php?t=113551
Check what he has done in order to color the crosshair.