I found some instructions that were for Gimp users and I want to do the same in PS and I am a bit of a rookie. The first instruction was:- Colours-> Components->Decompose RGB. This I believe gave a monochrome image and three layers Red, Green and Blue. The other instruction was:- Select a layer and add a layer mask greyscale copy. There are some more items which I think I can manage.
Ctrl+click on the thumbnails in the chanels palette, then create suitable layers from the selections.