Negatives and switching image colours in Photoshop

Photoshop Replace Colour

I quite often forget how to switch between a negative and positive image or how to do an exact colour replacement when using Adobe Photoshop. Each time, I need to, which is not very often, I spend a bit of time working it out. As this has just happened to me, I thought I would make a note of the steps and make that into an article. I needed to switch colours in a 2-colour logo to from blue on a white background to white on a blue background. This would have been fine if the coloured image parts were on separate layers or if it was a vector graphic