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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