Selective color PS to GIMP

[sallyanne]

I'm sure you have already looked through everything. I could only find a few things that might be what you are looking for.
In Gimp - color balance or Hue-saturation (the saturation) or Levels, Curves, and
In GMIC-Qt - CMYK Tone or Mixer CMYK
I'm sure someone will come up with a better answer for you. Maybe someone who knows photoshop.