Object item.growthcraft_milk.milking_bucket_iron has the texture of full bucket with milk. But it actually empty and used to get the milk from Growthcraft
When you spill milk from Milk Bucket you get empty bucket from Minecraft, but it will be more correct if you get item.growthcraft_milk.milking_bucket_iron after emptying Milk Bucket
Object item.growthcraft_milk.milking_bucket_iron has the texture of full bucket with milk. But it actually empty and used to get the milk from Growthcraft