The Lajnah ad-Da’imah was asked the following question:
Q: What is the ruling on someone who lets some of their hair grow long but shaves part of their head?
A: All praise be to Allah Alone, and peace and blessings be upon His Messenger, and his family and Companions.
It was related by Abu Dawud on the authority of `Abdullah ibn `Umar (may Allah be pleased with him) that: The Messenger of Allah (peace be upon him) forbade Al-Qaza‘ (shaving part of the hair on the head and leaving another part unshaved), and said, “Shave all of it or leave all of it.” The author of [Sharh Al-Iqna‘] said, “Al-Qaza‘ includes shaving parts on the side of the head, shaving the middle and leaving the sides – as most Christians do – shaving the sides and leaving the middle – as many foolish people do – or shaving the front and leaving the back.” Imam Ahmad was asked about shaving the back of the head, and he said, “That is what the Magians do, and whoever imitates a people is one of them.” Accordingly, it is known that it is not permissible to let some parts of the hair grow longer than others.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.
A Side Note: As for if it is a light fade which is hardly apparent, then that is permissible, as Ash-Shaykh Saalih AaliShaykh, may Allaah preserve him, said:
قصّ بعض الشعر بدون اسئصال فإن كان شبيها بالاستئصال تزاد الكراهة , و إن كان أقلّ و إنّما يكون بعض الشعر أكثر من بعض بقليل فإنذ هذا ممّا يعفى عنه , لأنّ هذا لا يشبه تقزّع السحاب و اختلاف شعر الرأس ما بين الحلق و ما بين غيره .
… [As for] cutting the hair without trimming to the roots, if it resembles cutting to the roots [of the hair] then it is severely disliked and if it is something less and it is merely having some hair slightly longer than other parts [of the head] then this is something which is pardoned since this doesn’t resemble the qaza of the clouds [in its splitting] nor does it have the hair of the head being made different from one another by the means of shaving or its likes.