To control the lighting of the shadows in the described method by wabe is the easy way to go, while using any of the available lighting models (like standard, global illumination, ...).

If you calculate with the Global Radiosity lighting model selected.
This already lightens up the shadows more naturally, because here in your case, it renders the light which in reality will be reflected from the house wall paint or ground colors.
And - right next to the Global Radiosity switch is a data field called 'Gain'.
Raising the value there for instance from 0.00 to 1.00 or some higher or lower, will influence, how much brighter or darker the shadows will be calculated.