Unable to reduce the saturation of banners and leather armor

I’m trying to create a resource pack that reduces the saturation of banners and colored leather armor pieces (aka makes the colors less vivid).

I’ve tried to reduce the opacity of the overlay textures and while that kinda worked with banners it had no effect on armor. I’ve also tried changing the general map colors in the optifine configuration file color.properties since in the documentation it says that it should also affect the banner colors (but that doesn’t seem to be the case).

If you happen to know how the game (or optifine) applies color to these entities please explain it as it would be a great starting point.