Copy content from paint layer to its mask
I ran into a case where I wanted to copy the content of my paint layer then create a fill layer and paste the paint layer content in the mask channel. Even if it was originally color it could still be pasted as gray scale into the mask channel.
Jeremie Noguer commented
You can already do this with Anchor Points:
1. Create an Anchor Point effect on your layer
2. Create a Fill effect on your mask and choose the Anchor Point in the minishelf that pops when selecting the content of the mask fill.
3. Select which channel you want to use as mask