As you can see, if you create custom biome with a datapack and use custom leaves colors, it does not affect all leaves.
Jungle leaves, oak leaves, dark oak leaves and acacia leaves works well, but spruce leaves, birch leaves and azalea leaves don't.
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Thank you for your report!
We're tracking this issue in MC-3930 and MC-213987, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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