Yes. You can change virtually anything about a drop shadow -- size, offset, color or blending mode.. -- from the definition dialog for the shadow. Object > Effects... and choose drop shadow.
Thanks Peter, but I need you to lead me a bit more on this. In an attempt to change the shadow color to white I did the following:
Settings for Fill>
Blending Mode> Colour>
Pick white swatch
This appears to have no effect on the shadow color.
If you pick white for the shadow you'll need to change the blending mode to something other than multiply, probably screen.