Highlighting ring - that's the data\textures\#green_circle.tga graphic if memory serves. Might have to unpack the file (see unpacking individual files) and then edit it.
Else (more likely) it's in descr_ui_color_db.txt - in the 'Battle Components' section, most likely these lines:
Code:
marker_proxy_summer_selected 255, 0, 255, 0
marker_select 255, 0, 255, 0
The first number is the 'alpha' code, eg how translucent the color is, the next three is the RGB code: 0,255,0 is totally green. Make sure to use a kingdoms version or from my Bare Geomod.