Is my action considered as zihar if I commented that my wife’s skin tone is the same as my mother’s?
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
Zihar is defined as husband’s words that equates or likens his wife that he has yet divorced through bain with a woman who is mahram muabbad for him. 
It is named as zihar because it refers to the act that originates from the words of a husband who likened his wife’s back with the back of his mother. The equating aspect made is as though the wife is prohibited for him to have intercourse with just like it is prohibited for him to have intercourse with his own mother, sibling or other mahram.
Zihar is also defined according to each madhhab as:
- Madhhab Hanafi: A Muslim who compares and likens his wife even with the People of the Book, small child, insane person or state similarities of a certain characteristic that is famous for her with another woman who is forever prohibited for him. 
- Madhhab Maliki: A Muslim who verbalize a resemblance of something that is originally permissible for him of his wife or slave or a part from them with his mahram. 
- Madhhab Syafi’e: Equating his wife who isn’t divorced through Talaq Ba’in with a woman who is impermissible for him to marry. 
- Madhhab Hanbali: A Muslim who equates his wife or any parts of her body with the back of a woman prohibited for him to marry forever or with any of her body parts. 
The ruling of zihar is it is prohibited according to the evidences from al-Quran and ijmak of scholars. Allah SWT states:
الَّذِينَ يُظَاهِرُونَ مِنكُم مِّن نِّسَائِهِم مَّا هُنَّ أُمَّهَاتِهِمْ ۖ إِنْ أُمَّهَاتُهُمْ إِلَّا اللَّائِي وَلَدْنَهُمْ ۚ وَإِنَّهُمْ لَيَقُولُونَ مُنكَرًا مِّنَ الْقَوْلِ وَزُورًا ۚ وَإِنَّ اللَّـهَ لَعَفُوٌّ غَفُورٌ
Those who pronounce thihar among you [to separate] from their wives – they are not [consequently] their mothers. Their mothers are none but those who gave birth to them. And indeed, they are saying an objectionable statement and a falsehood. But indeed, Allah is Pardoning and Forgiving.” 
- If the pronouncement is followed by talaq, then the zihar ruling is included in the talaq ruling and the zihar doesn’t affect anything.
- Consequently, if the zihar is not followed by talaq and nothing happens that can break the marriage, then he is considered to have gone back on his word, contradicting the meaning of zihar. Thus, he is fined with kaffarah and it should be paid off quickly.
According to the above question, the act of equating the skin tone of one’s wife with his mahram muabbad is not included as zihar. The reason is the comparison of similarity between the wife and mother in terms of the shade or tone of the skin is not considered as not physical characteristics. Hence, it is not considered as zihar. However, we advise to avoid such acts for there is concern that it could lead to zihar which is clearly prohibited in Islam. We also advised to leave such a doubtful act, so that in living our lives of practising Islam, we’ll attain calmness. This is as stated by al-Hasan bin ‘Ali R.Anhuma, where Rasullullah PBUH said:
دَعْ مَا يَرِيبُكَ إِلَى مَا لاَ يَرِيبُكَ فَإِنَّ الصِّدْقَ طُمَأْنِينَةٌ وَإِنَّ الْكَذِبَ رِيبَةٌ
“Leave what causes you doubt and turn to what does not cause you doubt. Truth is tranquility, but falsehood is doubt.” 
Lastly, may Allah SWT grant us a clear understanding in religion. Amin.