[ground_segment] fix green->darkgreen change

OMG, I finally upgraded from Ubuntu 12.04 and noticed that the green color changed and made everything less readable...

Since tcl8.6 "green" refers to same color as "darkgreen" (to the one used in web) and the former "green" is now "lime"...
But "lime" is not available in older versions, so hardcode the color to `#00ff00`