I have a black layer with transparency where I want to mask out the layer below. I can reverse this if needed. I remember using luma matte to mask a layer underneath a solid black or solid white layer, but I cannot seem to remember the right way to do this. So how can I us a psd layer to mask a layer or comp that is below it?
Put your black and white layer (we'll call it A) above the layer you want to mask (B). Set the Track Matte (shows up as TrkMat in the timeline panel; you may need to hit the "Toggle Switches/Modes" button if you don't see it) on layer B to Luma Matte. Anyplace that's black in A will be transparent in B, anyplace white will be visible. Anything gray will give partial transparency.
Now, if A is a black layer with transparency, you'll want to use an Alpha Matte. Same principle, but anyplace that's opaque will be visible in B and anything transparent will likewise be transparent. If you're going Alpha Matte, the opaque area color is irrelevant. It could be black, white, rainbow colored, a lovely photo of a sunset, doesn't matter. All After Effects looks at is opacity.
It is easier to destroy than to create.
More fun, too.