If you need a setup that allows you to change to a large number of different colours, we'd do it differently.
The Channel Mixer layer would simply have the "Monochrome" box checked, and +100, 0, 0, 0 as the values.
Then you'd add a Solid Color layer on "Multiply" blend mode. That's the layer whose colour you could change to anything you want. Start with blue, and fiddle with it later.
The trouble with this method is that the masking needs to be much more precise. The mask can still go into the hair a little bit, but nowhere near as easily.