I used a pic of the LLoyds building and duplicated it so that it was on the first layer. Then I clicked the New Layer button and filled it with a mid blue. I drew mad squiggles on it with a paint brush and went over them with the Impressionist brush. Thought this would make a change from a gradient.
Clicked on the middle layer and went to Filters, Adjustments menu and selected Invert. The same as PSP Negative Image.
Then had a play with the layer palettes Modes menu. I chose Difference for the top layer and played for ages with the middle layer options. All worked well.
Finally I went back to Filters and Adjustments and played with the gradient map. Hours of fun, here.
Hope that helps a bit with the Layers conundrum.