Why did the tuktok of Rapunzels hair turn brown in Tangled?

It is shown that once her hair is cut it turns brown and loses its power. However, not all her hair would turn brown, only what was cut. So when Flynn cuts Rapunzels hair in the end, it would make since for where the cut was and down to turn brown, but not from above to her head. The tuktok of her hair should have stayed blonde.