I want to find out from you guys if its normal for the avent bottle to turn brownish in color after 4 months of use?l have used this bottle for my 8 months old son, l love the new teats he adjusted to them just fine.The only problem l have is the color change of the bottle.l tried purchasing some other brands but nothing is as good as the Avent bottle but then again l have doubts in purchasing this product as l'm worried about the bottle turning brown again.