When you create a mask, the image window shows the effect of the mask,
but does not display the mask automatically. Use the Mask Overlay
toggle 
on the Layer palette to display the mask overlay.
By default, the overlay is a 50 percent opacity red color for completely
masked pixels and lower opacity red for partially masked pixels. When
you paint or modify the mask, this overlay shows what is painted or changed.
To change the color and opacity of the mask overlay, see Changing
the Mask Overlay Color.
When the mask overlay is displayed, the Mask Overlay toggle displays
. When the overlay is hidden, the Mask Overlay toggle
displays 
.
To display or hide the mask overlay:
Click the Mask Overlay toggle 
, or choose Layers > View
Overlay.
Creating Masks from Selections