Sin is decided by God, not by us. If God says it is sin, then it's sin. If God doesn't say it's sin, then it's between us and God. God says, lying is a sin. I don't get to decide if it is or isn't. God doesn't say a thing about painted fingernails being sinful. Then that's between me and God. Others DO get to tell you if you are sinning. And the church can excommunicate you if you do.
James 4:17
So whoever knows the right thing to do and fails to do it, for him it is sin.
New International Version
It would have been better for them not to have known the way of righteousness, than to have known it and then to turn their backs on the sacred command that was passed on to them.
So, sin is up to you to decide.
You or the church judging others is a different matter:
Matthew 7
“Judge not, that you be not judged. For with the judgment you pronounce you will be judged, and with the measure you use it will be measured to you. Why do you see the speck that is in your brother's eye, but do not notice the log that is in your own eye? Or how can you say to your brother, ‘Let me take the speck out of your eye,’ when there is the log in your own eye? You hypocrite, first take the log out of your own eye, and then you will see clearly to take the speck out of your brother's eye.
Luke 6
37 “Judge not, and you will not be judged; condemn not, and you will not be condemned; forgive, and you will be forgiven;
John 7
24 Do not judge by appearances, but judge with right judgment.”
James 4
Do not speak evil against one another, brothers. The one who speaks against a brother or judges his brother, speaks evil against the law and judges the law. But if you judge the law, you are not a doer of the law but a judge. There is only one lawgiver and judge, he who is able to save and to destroy. But who are you to judge your neighbor?
Romans 2
Therefore you have no excuse, O man, every one of you who judges. For in passing judgment on another you condemn yourself, because you, the judge, practice the very same things. We know that the judgment of God rightly falls on those who practice such things. Do you suppose, O man—you who judge those who practice such things and yet do them yourself—that you will escape the judgment of God?