The Guardian:

If you have a suspicion that supermarkets have been subtly changing the colours of milk bottle tops, then you are a more observant person that I am, and you are right. The green colour of the semi-skimmed milk bottle tops has become an issue for materials engineers over the past few years, and it is they who have been putting pressure on the supermarkets to change it. The reason is not that they want the colour to better reflect the green meadow grass that cows eat, nor is it that they have discovered a shade that makes the milk taste creamier; no, it is because the green pigment of semi-skimmed milk bottle tops affects the ability of the plastic to be recycled.