ok so you go to the clangen folder (NOT the data directory one) then sprites then dicts there's a JSON file called white_patches_tints. Open it and scroll to the bottom to the "tint colors" section, then to add a new tint you just add a new column (example: "dark": [38, 38, 38],) remember to add a comma at the end. the numbers are the RGB values for purple this is 128,0,128
To make sure the new tint actually shows up you have to add it to the "possible tints" section, just above the "tint colors" section. this basically decides which color group the tint will appear on (so a chocolate cat will have tints from the basic and brown groups while a sienna cat would get them from the basic and ginger groups)
save your changes and generate some cats to see if it worked. backup the original version just in case.
Tell me if this is understandable?