When using

{
    "parent": "item/generated",
    "textures": {
        "layer0": "items/fish_egg_changer",
        "layer1": "items/pufferfish_egg"
    }
}
   

in

 assets/minecraft/models/item 

colors of those two, and only those two spawn eggs are diffrent than in pngs. Even if color saturation is set to 0, those two textures have blue color (green instead of blue for pufferfish because of yellow color in png)