I first selected a photo I thought would work with selective colouring and made a copy. The copy I then converted to black and white.
I uploaded the coloured version into PicMonkey and went to "Overlays". In overlays I selected "Add your own" and uploaded the black and white copy of the same photo. I sized it to fit over the original and then selected the "eraser". I carefully removed the area of black and white I had decided to use as the selected colour section.
So simple.