Set layer color based on other layers

[Hellcolik]

Hello, this is my first post here ; I'm wondering what are the limits of Gimp without using external python scripts.

I have two layers in a project:

• the back layer is a picture on which I applied a Gaussian blur filter ;
  
• the front layer in only a text.
  

I would like the text layer to change depending on where it is above the back layer. For instance, I would like to order it : "to set the color of this pixel of yours, look at the color of the pixels on the layer behind you, centered at the same location, take the negative of that color and apply it to yourself".

What are the tools/menus I need to be familiar with in order to perform such a task? I would be able to do this with a python script however, I only want to use Gimp.

Thanks in advance, I'm looking forward to reading your answers.