Dear aspirant

Usually they will match your face/apperance with the admit card photo. Photo on admit card will be the one which you uploaded during your neet application form which requires the latest photo of yours name and date of photo written on it.

Changing hair color will be not much problematic but you should appear same as photo so if you are doing Lil change then not a problem. If your changing your whole hair color which will make your appearance totally from the photo you provided then in that case you might face some issue. Your  examiner should be convinced that you are same person which is on your admit card photo.

So think wisely and then you can change although a month left for neet exam so you can wait till then, why to take risk. After neet you can make changes whatever you want freely.

Hope this was helpful

All the best