ft-CombineRGA Pixel Bender plugin allows you to shift channels or combine channels from a second source. What makes this plugin special versus the similar plugins that comes with After Effects, is that it works in 32bit and has a very simply to use UI.

Here are two presets knocking out black OR white from a layer based on the luminance to set it as transparent/alpha.

The Difference Matte effect creates a transparency a mask or a matte by comparing the pixels in a source layer with the pixel in a defined difference layer, and then keying out (eliminating) pixels in the source layer that match both the position and color in the difference layer.