Today we are very familiar with squeezing paint out of plastic containers or metal tubes, just like we do when we squeeze toothpaste out of a tube. When Millais was painting Ophelia, lead or tin collapsible tubes to store paint in had only just been invented. Tubes today that contain readymade oil paint are made from aluminium alloy or plastic.
Before the invention of tubes, painters would have carried bladders (see image above) to store the paint that they would have made in their studio. The bladders would have been made from pig membrane and tied at the top with strong twine to exclude air.
Millais may also have carried paint in jars (see image above) or tins, which would have made his carrying case quite heavy. Collapsible tubes were popular not only because they provided the artist with ready-made paint and therefore saved preparation time, but because they were also much more convenient to carry. The more you used them the smaller and lighter they got!