If you turn off the visibility of the yellow layer, this is what the blue layer looks like by itself.

Notice that the blue image icon in the layers palette is still solid blue. The pixels of the M are still there, but are being blocked by the mask.

If you turn off the visibility of the yellow layer, this is what the blue layer looks like by itself.

Notice that the blue image icon in the layers palette is still solid blue. The pixels of the M are still there, but are being blocked by the mask.