Aladdin for Artists? Maybe. I need some very small quantities of infrequently used oil colours to take with me on the Canadian Rockies trip in early August. I have some big empty tubes but no tiny ones. Here is how I tackled the problem.
I had some old Reeves small tubes of watercolour - so old that they were beginning to harden inside the tube. I cut them in half so that I had the cap, top and about 1 or 2 centimetres of the body left (I pressed the remaining colour into homemade pans). It was an easy matter to clean out the tube and cap with a penknife and warm water. I then got some 60mm wide sticky aluminium tape - I think I bought mine for repairing a gutter - and cut off a short length. I peeled off the backing but left enough so that when the tape was rolled round the remains of the tube, the backing acted as a lining. Then I rolled the tape round the old tube and used the body of an old biro to help re-shape the tube to roundness. Now I had an empty tube.
With the cap on I filled the new tube body, using a piece of doweling to push the oil paint firmly into the tube so that there were no air gaps (you can take the cap off to allow some of the paint to escape thus ensuring a good fill). I folded the end of the tube in the normal manner and labelled it.
The following picture shows, from left to right: sawn off and cleaned old tubes, Aluminium tape, new empty tubes and new full tube. The whole operation takes about 20 minutes and you waste some paint but if weight and bulk are an issue then it is worth it.
You can use just about any old tube that can be cleaned out: I have recently used balsa cement tubes. The full tube in the picture is one from a watercolour set I bought in Hong Kong in 1975! I don't suppose that the paint will last forever in these makeshift tubes but they will be very convenient when I am carrying everything up a mountain.