As an alternative to changing the color, you could add some sort of border or back-lighting effect to pop out the silhouette.
It's faster and easier than adding a real background, but it has a similar effect on the foreground, and backlighting can imply a complex background, left undisplayed.
You'd need the keep the lighting subtle, if you went that way, so as to not overwhelm your shading. A little bit of red glow, from behind the upper right corner, would really make him easier to see, without messing up the shading.
--or you could just do what I do and put a random streak of color back there.
That always makes people think of low budget Japanese cartoons though...
