As per your symptoms and if your teeth were never traumatized there can be two conditions. First that there could be staining of tooth by some stain causing food items or beverages like spicy food, tea or coffee etc and second could be dental fluorosis caused due in intake of water with high fluoride content during age of teeth development in body. People living in some areas of haryana, UP, MP, Rajasthan and himachal usually develop dental fluorosis causing greyish or yellowish discoloration of teeth and may sometimes even changes the shape, size and texture of teeth. It is highly recommended in your case to visit a nearby dentist for the solution.
You may have to get scaling, bleaching, veeneers or aesthetic zircinia crowns ....
I also advice you to stop regular use of mouthwash if you do, because mouthwashes also cause discolorations in teeth on overuse.
Transparent or greyish. patches can be a intrinsic stains .. these can be also from the location you are from its called "flurosis ".which might require veneers , rct , crowns . Try flossing if it is external stain it will go .
It's difficult to give a solution without examining your tooth clinically.. Tooth stains or discoloration can be of two types.. Extrinsic or external which can be removed by scaling and polishing.. Internal or intrinsic stains which develop inside a tooth, needs to be examined and treated by root canal and crowns, if required, or bleaching.