Hi all, pretty dumb question here. Sorry about that.
How do I copy empty space in Photoshop? Here is what I mean. I have this so far:
Layer 1: colour map
Layer 2: copy of Layer 1 but in monochrome
Layer 3: copy of Layer 2 with the 1st area of interest erased. (It's a river basin I'm going to be presenting a talk about and showing pictures of.)
Layer 4: copy of Layer 2 with the 2nd area of interest erased. (Another river.)
Layer 5: copy of Layer 2 with the 3rd area of interest erased. (The third river.)
Now I can, for example, turn off all layers except 1 and 4, and "save visible". This provides a monochrome map with the 2nd interesting river valley highlighted in full colour: exactly what I want to illustrate the part of the lecture about that river. Or turn off Layer 4 and replace it with Layer 3 to have the first river valley showing in colour against the greyed-out background, and so on.
Finally - and here is where the problem crops up - I want the grey background but with all three rivers in colour at the same time. This doesn't work! It just looks like Layer 2 and everything is grey. I could copy Layer 2 to make Layer 6 and re-draw (well, re-erase) all of the rivers. That would certainly work, but surely there is a way to get Photoshop to do it for me? What I need is a copy of Layer 2 minus the erased parts of Layers 3, 4, and 5. I imagine that there is a simple way to do this, I just can't think of it.