Pseudo-infrared

[Ofnuts]

Explaining something I obtained something that looks like an IR photo, so here it is...

The initial question is about inverting luminosity while keeping the colors...

So:

• Put your image in a group (bottom group in screenshot below)
  
• Add a white layer below it
  
• Set the image layer to LCH Lightness mode
  
• The result of the group is a grayscale version of the image
  
• Layer > New from visible to "freeze" that result
  
• Color > Invert for a negative version
  
• Add a second group (top group in screenshot)
  
• Add a copy of your initial image in it
  
• Add the inverted layer above it in the group
  
• Set the inverted layer to LCH Lightness mode
  

   

Edit: there is even a way to avoid the New from visible (layer modes in parentheses when different from "Normal"):

   

[Tas_mania]

Looks good. It reminds me of the 'solarize' effect that I use sometimes.

[denzjos]

Nice and easy to go back :
Colours / Hue-Saturation : hue slider to the left
Colours / Invert