Say I just want to apply an effect on the green channel of a layer (like I want to mask and paint just on the green channel for chromakey work) Is there a way to split out a color channel, apply an effect, then recombine the channels? For node based compositors, this is a snap. But how would I do this in AE?
Splitting and recombining the channels is easy enough if you don't mind working with one layer per channel. To split them, use any number of effects that can adjust individual channels (Shift Channels, Levels, Curves, etc.). To recombine them, set the top two channel layers to Add blend mode.
Of course, things get trickier when there's transparency involved.