Get white object to turn black when over white

juliasjunkyard.files.wordpress.com/2012/06/blackwhite4.jpg

I would like to figure out a way to have the white objects turn black when they're over something white as seen in the link. You can see the snow is both black and white when it both over the black sky and the white moon.

Any ideas?

I think the easiest way to do this is by using collision check,

as when the snow on collision with the moon, change to another frame, or use shader effect to change the color,

though I'm not sure how it will impact the performance

Here your best bet would be blend modes , check out the template that comes bundled with C2 !