Hi Simon,
What I did before decolourising was to crank up the magenta colour to the max using variations. Then Igave it a boost in contrast and brightness. Sharpen to get a bit more detail and then a small amount of smart blur to fuzz the left and right sides. At the end I had to darken and blur the top of the yellow gate to get it to behave itself. Is this anything like the effect you were looking for?