Hence some of the salaf used to say, ‘If a person of innovation were to dye his beard every day, the dye of innovation would remain on his face.’

“The person who is righteous and honest, his honesty is manifest from the radiance on his face, and his honesty can be known from the glow that is on his face, likewise the (opposite for the) sinful one and the liar. The older a person gets, the more this sign becomes apparent. Thus a person as a child would have a bright face, however if he becomes a sinful person, adamant on committing sins, at the older stages in his life, an ugly face would manifest that which he used to internalise, and the opposite is also true.

It has been narrated that Ibn Abbās (رضي الله عنه) said, ‘Indeed righteousness illuminates the heart, radiates the face, strengthens the body, increases provision, and produces a love in the hearts of the creation for that person. Whereas sinfulness darkens the heart, greys the face, weakens the body, and produces hatred in the hearts of the creation for that person.’

It is possible that a person may not intentionally lie; he may even be a person who makes great effort in ibādah and has zuhd (abstains from pleasures of this life that are lawful). However he has false, incorrect ʿAqīdah regarding either Allāh, His dīn or His Messenger ﷺ or His righteous servants. And what is on the inside affects what is on the outside. Thus, this false, incorrect ʿAqīdah that he thought was true and correct reflects on his face, and his face would be dark in accordance with the level of falsehood he possesses. As it has been narrated that ʿUthmān ibn Affān (رضي الله عنه) said, ‘No one ever hides evil within themselves except that Allāh makes it manifest from his facial outlook and the statements his tongues utters.’ Hence some of the al-Salaf al-Ṣāliḥ used to say, ‘If a person of innovation were to dye his beard every day, the dye of innovation would remain on his face.’ On the day of judgment this would be very clear as Allāh says,

وَيَوْمَ الْقِيَامَةِ تَرَى الَّذِينَ كَذَبُوا عَلَى اللَّهِ وُجُوهُهُم مُّسْوَدَّةٌ ۚ أَلَيْسَ فِي جَهَنَّمَ مَثْوًى لِّلْمُتَكَبِّرِينَ ‎﴿٦٠﴾‏

“And on the Day of Resurrection you will see those who lied against Allāh their faces will be black. Is there not in Hell an abode for the arrogant ones?”
[Al-Zumar, 39:60]

He taʿāla also said,

يَوْمَ تَبْيَضُّ وُجُوهٌ وَتَسْوَدُّ وُجُوهٌ ۚ فَأَمَّا الَّذِينَ اسْوَدَّتْ وُجُوهُهُمْ أَكَفَرْتُم بَعْدَ إِيمَانِكُمْ فَذُوقُوا الْعَذَابَ بِمَا كُنتُمْ تَكْفُرُونَ ‎﴿١٠٦﴾‏

“On the Day when some faces will become white and some faces will become black; as
for those whose faces will become dark (to them will be said): “Did you reject Faith after
accepting it? Then taste the torment for rejecting Faith.”
[Alī Imrān, 3:106]

Ibn ʿAbbās and others have said regarding this verse, ‘The bright faces will be ahlussunnah, and dark faces will be the people of bid’ah and division.’”

Source: ‘Al-Jawāb al-Ṣaḥīḥ’ (Vol.4, pg. 306-307)