pay close attention to halo head number 1 in my sig.
see the shapes? they are either separated by
1. a very faint and blurry line (see the details on the top of the head/the details on the visor)
2. a very faint change of color (see the faceplate/mouthguard)
not a very drastic change in color or not a very dark and rigid line in sight, as i said, just enough to let you distinguish the shapes from one another.
i always say, outlining is very a bad practice in texturing, and i aims to be rid of it as much as possible
put more effort in the transition areas of your shapes
also, try this experiment. instead of putting outlines, to mark your shapes, use a drop shadow, you'll see what i mean