Adding Layer Mask to Hue/Sat Layer (Multiply Blending Mode?)

You can see my layers in the photo attachment below.

I adjusted the curves in the Hue/Saturation layer so just the illuminated slopes of the mountains (layer "DEM") are yellow.

About the 3rd Layer:
I think I would like to create a layer mask (out of the layer 3rd from top) to add to the top layer. My hope is that the black in the 3rd layer will not allow the yellow to be seen as much. On this layer the black represents lower elevation and the white represents higher elevation.

My Goal
If just the mountain slopes at higher elevations are illuminated with yellow tint, it gives the effect of alpenglow, or sun falling on high peaks before it sets completely.

How would I go about doing this? A tutorial suggests Multiply Blending Mode, but I'm not sure if this is right because I haven't succeeded. Click the tutorial link and scroll down to "Additional Techniques" to see the example.

Any help or walkthrough is greatly appreciated. I'm a geography student, and I haven't found a solution with my photoshop books. Thanks in advance!

Click here and scroll to "additional techniques"