Tip from HeroQBE in minecraftforum.net, possible workaround:
"For editing e.g. birch leaf color create or edit the file birch_leaves.json in the folder YourResourcepack\assets\minecraft\models\block\ and edit it like that:
That will just turn off the block tint and you can change the leaves_birch.png in the YourResourcepack\assets\minecraft\textures\blocks\ folder like you want 🙂 "
Kaari Koehn
Tip from HeroQBE in minecraftforum.net, possible workaround:
"For editing e.g. birch leaf color create or edit the file birch_leaves.json in the folder YourResourcepack\assets\minecraft\models\block\ and edit it like that:
{{{}}
"parent": "block/cube_all","textures": {"all": "blocks/leaves_birch"{{}}}
That will just turn off the block tint and you can change the leaves_birch.png in the YourResourcepack\assets\minecraft\textures\blocks\ folder like you want 🙂 "
I haven't tried this yet myself.