This is an Adobe Swatch Exchange file which contains swatches for all greyscale colors which can be safely exported by the NVidia Texture Tools DDS exporter without discoloration/tinting due to DXT compression.
To install, simply open the Swatches panel in either Photoshop or Illustrator, click the menu button in the top right corner of the panel, and select "Load Swatches" or "Open Swatch Library > Other Library" and select the provided file. Use any color in the swatch group for greyscale colors in your textures.
For more information, see the Texture Compression guide:
Swatch palette configured by PNWParksFan
Original compression comparison chart by CJ24