After making the RGB channels visible again by checking the little RGB box in the channels palette, I took a black brush and painted an M on the mask. Notice that the mask icon is selected in the layers palette and the mask channel is visible. You can see the M on the mask icon, but the blue image icon remains solid blue.

The M looks orange, not yellow because the layer mask channel is set to be visible. It displays as red at 80% opacity in this example. Having the layer mask visible allows you to see the content of the layer (all blue) simultaneously with the mask. To just see the effect of the mask, click the little box in the Layer 2 Mask channel to suppress the mask channel display.

After making the RGB channels visible again by checking the little RGB box in the channels palette, I took a black brush and painted an M on the mask. Notice that the mask icon is selected in the layers palette and the mask channel is visible. You can see the M on the mask icon, but the blue image icon remains solid blue.

The M looks orange, not yellow because the layer mask channel is set to be visible. It displays as red at 80% opacity in this example. Having the layer mask visible allows you to see the content of the layer (all blue) simultaneously with the mask. To just see the effect of the mask, click the little box in the Layer 2 Mask channel to suppress the mask channel display.