Some time before 1984, LEGO changed the formulation for the plastic used in some of the more flexible parts such as clip plates, minifig hands and airtanks. The color was noticeably affected, with the earlier parts having a “milkier,” more translucent color than the newer parts, which have a more consistent flat coloring.
The difference can be seen in the image here. In each pair, the newer version is on the left, the older on the right. Only these four colors appear to have the variations. Blue and black airtanks, which first appeared in sets from 1984, do not seem to have been produced in the “softer” plastic. This gives us the clue that the change happened prior to 1984. Because LEGO usually continues to use older variants in new sets until the stock is depleted, it is possible that sets after 1983 included these older tanks.
The difference in color is most pronounced with light gray. The older light gray tanks are considerably darker than the newer ones.
The location of the mold pip also changed on this part. At first the pip appeared on the back of the right tank near the bottom. Later it was moved to the top of the part. Because the tanks appear in the newer color with the mold pip on the back, we know the mold change happened after the change in plastic formulation.