The colour differences are not so critical here. You have a lot more freedom with this image to create some quite wacky variations. As long as the Saturation was not too high, I found I could have virtually any colour combination for the wood and rope.

It's an interesting point though, because some subjects are not so tolerant. The girls hair for example, only allowed light colours to replace the original white background. Darker colours created undesirable effects on many areas. This is just the nature of how hair will photograph with different backgrounds and lighting situations. Knockout leaves transparency in some areas for a more natural blend with the host image, but this won't work well in all situations. Ideally if you were taking the original photo yourself you would shoot with a background and lighting that simulated as closely as possible the host image lighting.

Still playing with Knockout. No doubt I've still a lot to learn to get this most out of this app. Still, very impressed so far.

Sark