Dark Golden Retriever puppies that have full, solid colored coats from root to tip often retain the color specifics in adulthood. Puppies with different colored undercoats or hair with different colored tips tend to change coat color as they grow.
If the puppy is a pure-bred, a good indication is to look at the puppy's parents. Very little changes from the parents will occur as the pup grows older. Most changes in the coat will occur within the first year of the pup's life. Most changes from the pup as it grows older will be with the length and texture of the fur, rather than in the coloration.