Thank You for this question:
I will try to explain, as this can be a little confusing.
Certain rules need to be followed for this to work.
Apply effect in viewport first:
Add effect to the list:
To adjust it in viewport set Material rollout "From" and "To" to 100.
"Altitude 0" is pivots position- Pink box height in my example:
I adjusted Altitude 50m and Range 50m as well:
Render is totally unexpected:
Lets change forest material:
3 important settings here:
Slider set all the way to items
Blending mode Normal (I recommend this but once You are done with all the steps You can return to play with other modes)
Render: now we are getting somewhere:
Important: think of Tint by altitude as Mask by altitude- it is what You want in the first place
Let setup material further using forestmap as mask:
We can replace simple color to new forest material with total control:
And final demo render:
Hope that helps.
Demo scene included as well for closer inspection should You need it.