Hello.

Let me tell u this very clearly they r going to check ur specs also so be prepared for that and dont panic. And shirt,wear plain shirt of any color,that would be better. And i dont think there is any rule for color. Dont were belt or tie or sacred thread they might ask you to remove it. Go with admit card and id proof and ur photo. Pen they will provide. Rest anything they will not allow. 

For complete guidance go through this article below

https://www.google.com/amp/s/medicine.careers360.com/articles/neet-rules-and-regulations/amp

Hope this is helpful.

Good luck.