Invert intensity (HSI)

Playing a little I found the XF86VidModeSetGammaRamp() function that allow to change the LUT of the rgb channels.
The problem is that's useless...
I want to invert the intensity, but leaving the colours as they are (so light blue will become dark blue instead of dark orange)
┬┐any ideas?
well this could be done in openGL, something like

glBlendFunc(GL_ONE_MINUS_SRC_COLOR,GL_ZERO);

maybe make a special RGB cable with the signal/ground swapped over for each analog color channel? Don't know if that would work!
No, that would simply invert the image. (green would become magenta)
I just want to touch the intensity.

Also, I'm using ratpoison so could not make an `always on top' window
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