Setting a key frame for the contrast/gain settings in Nuke

Setting a key to change the range values in Nuke.
For this shot the iceman melts in the foreground - it's the end shot of the melting sequence and I wanted to play with the opacity, so I set a key frame at the start (highlighted red rectangle on the left of the timeline below) and another at the end and changed the value sliders on the right.
Using the ID passes, I was able to isolate the men, by selecting the rgba_green channel in the ranges of the colour correct node, and apply the transformations.
Image below with full contrast/gain values

 Image below with the contrast/gain values adjusted, which gives the appearance of a fading colour