i also use pigeon and i also realized after some time the teats will turn white.
it could be:
- milk residue not washed clean and start to build up.
- method of sterilization. i do not use steam sterilization, but rather high heat sterilizer (like an oven) to sterilize that that also contribute.
change the teats regularly, i change mine on a 6 to 10 weeks basis.