Edit a layer mask

1. In the Layers panel, select the layer containing the mask you want to edit.
2. Click the Pixel Mask button in the Masks panel to make it active. A border appears around the mask thumbnail.
3. Select any of the editing or painting tools.
   Note: The foreground and background colors assume default grayscale values when the mask is active.
4. Do one of the following:

   • To subtract from the mask and reveal the layer, paint the mask with white.

   • To make the layer partially visible, paint the mask with gray. Darker grays make the level more transparent, lighter grays make it more opaque.

   • To add to the mask and hide the layer or group, paint the mask with black. The layers below become visible.

To edit the layer instead of the layer mask, select it by clicking its thumbnail in the Layers panel. A border appears around the layer thumbnail.

To paste a copied selection into a layer mask, Alt-click (Windows) or Option-click (Mac OS) the layer mask thumbnail in the Layers panel to select and display the mask channel. Choose Edit > Paste, then Select > Deselect. The selection is converted to grayscale and added to the mask. Click the layer thumbnail in the Layers panel to deselect the mask channel.

 community help (adobe.com)