This demo has two layers:

• one layer with an equirectangular image.
• above it, another layer with the same equirectangular image, but desaturated.

Initially only the top layer (desaturated) is visible. Editing its alpha channel makes the colored layer underneath visible.

• Shift+click: hide desaturated layer (alpha=0)
• Ctrl+click: show desaturated layer (alpha=1)