MRIcron stores color lookup tables in its /Resources/LUT folder. These files must have the filename extension .lut. These can be either ImageJ binary format (these files are precisely 768 bytes) or Osirix text format (these are larger than 768 bytes and can be modified with a text editor or Excel). The 'overlay_classic.lut' is the latter. This format stores 256 colors each defined by an brightness index (0..255), plus red, green and blue components (each 0..255). So to invert the color scheme just change the index column so that all values 1..255 are flipped (255..1). I would leave index 0 untouched (this makes the darkest voxels black). You can place the edited file' overlay_inv.lut' in your LUT folder and restart MRIcron.

https://github.com/CPernet/brain_colours...
https://gist.github.com/stowler/5493255
https://people.cas.sc.edu/rorden/mricro/...

While I am happy that my MRIcron software has proved popular and robust, my current development efforts have shifted to MRIcroGL. MRIcroGL leverages OpenGL 2.1 (released in 2006) and Python scripting. It also includes many color schemes (viridis, mako, plasma, inferno) that are designed for common forms of color blindness.
  https://cran.r-project.org/web/packages/viridis/vignettes/intro-to-viridis.html
  https://github.com/CPernet/brain_colours