Shader question !

Hello everyone.
For my round-based strategy game, I am going to create, I need a terrain in style of the total war games. The creation of the terrain is of course no problem, but I want to show the players/computers "empire" on the terrain. This should look like coloured transparent circles on the terrain, with a model as the origin. That means: if a certain model is placed on the terrain, it becomes the origin of a propably yellow circle on the map.
Is this possible with shaders? I do can script in .wdl but I have no idea of making this kind of effekt.
It would be nice if anyone can help!
greets

http://www.conitec.net/beta/ent_decal.htm

Maybe this is what you´re looking for.
They´ll bend over your hills and into your valleys.

thanks for this idea. I did not thaught of this command.
it would be possible for me to use it but a shader effect would be better.
The problem with this is: if the player builds a certain building, a circle bitmap gets attached to the terrain. thats fine. but if the player builds another certain bulding IN this circle, they both will reduce the alpha in the area where both bitmaps are visible.

is a shader effect like this even possible? again: i am a totally shader-noob :-S