Doug, there is no shadow-only color adjustment in TG, in part because shadows and shadow color are actually computed fairly realistically. But that doesn't mean you can't influence the color.
As you might imagine the color in shadows comes essentially only from bounced light, i.e. GI (aside from the actual surface color that the shadow is falling on, of course, but you know how to change that...). The Enviro Light controls the way GI interacts with the scene and has some parameters for biasing things. If you adjust Colour on Surfaces then you effectively tint the shadow color, although you also tint all other GI lighting on surfaces, which has a noticeable but more slight effect on non-shadows surfaces (because primary lighting contributes more heavily to color in non-shadowed areas).
So for example attached is an image where I made the Colour on Surfaces pure red and you can see that the shadows are notably red, especially noticeable on the lighter-colored sphere (where of course the color of the shadows is more obvious due to the light surface color).
Now of course this approach may give you results you don't quite want, e.g. color tint on normal surfaces too. But of course this is more realistic. If you only use a slight color tint rather than the extreme (full 255 red) I used here, then it's quite possible you won't even notice the effect on primary-lit surfaces and only in the shadows. So it's a useful thing to be aware of and experiment with.
- Oshyan