____How do I copy a Keyed area (using Keylight) from one Layer to another in the same project?

[Hugh Betcha]

How do I copy a Keyed area (using Keylight) from one Layer to another in the same project?

I have multiple versions of a colourful oject in the center of the frame, and I have a Cyan and black version of the footage, where I want to delete the Cyan.

I've done so, using Keylight, but I am unsure how to copy the 'mask' it makes to the other layers... is this even possible?

[Rick Gerard]

You can duplicate the layer (Ctrl/Cmnd + d) and use it as a Track Matte, or you can use the layer with Keylight applied as the Source for Set Matte to create transparency on another layer. 

A screenshot, embedded by dragging to the reply field instead of attaching, that shows us the layers in your comp and a little better explanation of what you are trying to do would help us help you.

[Hugh Betcha]

First is Magenta Background (so I know when the key works)

Second is subject (this version has no color/graphics, but you get the idea of what I want to crop)

Third is 'Mask' footage - basically the black is for the subject, and the Cyan is what should be 'see through'...

   

[Hugh Betcha]

Turns out your solution was good - the Keylight idea was wrong for this - the 'Effect-Channel-Set Matte' worked - Thanks Rick!

[Hugh Betcha]

Not so good, actually - when I move in the timeline, the mask doesn't move with it for some reason!

[Rick Gerard]

Pre-composing, paying attention to rendering order, and turning collapse transformations on and off can usually solve the problems you seem to be having. 

It would really help to see an uncropped screenshot of your comp with the modified properties of the problem layers showing. Press 'uu' and let us see what is going on.

Set Matte does not take into account layer transformations, so if you move a layer you want to use as a matte, you have to pre-compose it before you can use it as a set matte. Track matte may be a better option.