It is cheaper to buy artists paint in bulk jars than in tubes. One tends to waste more paint, though, because air gets inside the jars every time you open them.
I used to use a lot of silicone rubber that came in grease gun cartridges. After emptying the cartridges of silicone, I disassembled them for cleaning and storage, because they just looked too nice to throw away. Eventually, I hit upon this use for the empty cartridges.
I packed the empty cartridges with the bulk paint and made caps out of silicone rubber.
Since air can't get inside the cartridges, the paint lasts a lot longer. You just uncap the tip of the cartridge, squeeze out the paint you need, and cap the cartridge again. There is never any problem with dried paint gluing a screw-on jar lid tight, either. Being made of silicone rubber, which nothing sticks to well, the caps never get stuck. They just pull off and push on.
Step 1: Hang Up the Paint Cartridges.
Step 2: The Caps
Just take a new cartridge of silicone and build the cap on the tip of the cartridge. Extrude the silicone from an already opened cartridge and do any touch-up of the extruded silicone with a painter's palette knife. It takes a few hours to harden up. Pull it off and make more with the same cartridge tip. You will need one for each paint color's cartridge.
Step 3: Cleaning the Cartridge and Packing With Paint
The push rod helps clean out the nozzle. I made some more tools like the push rod to help with the cleaning. One has a bent tip for snagging and pulling the silicone from inside the cartridge. Another has some rag material wrapped to and tied onto the rod to help rug off what silicone is left inside.
Once clean, you can use a spatula to transfer paint from the paint jar to the empty cartridge. Cover the floor with newspaper. Keep the silicone cap on the end of the cartridge and tap it occasionally on the floor to prevent air pockets in the paint. Air pockets in the paint can compress later and continue pushing out paint after you stop pressure with the grease gun.
Fill the cartridge completely with paint before removing the silicone cap and replacing the piston cup, to avoid trapping air inside the cartridge. As you push in the piston cup with your thumb or a stick until it is flush with the end of the cartridge, paint will come out the nozzle end. Catch it in the old paint jar. You won't need to open the jar again until the next time you pack a cartridge, so it stays pretty fresh in the jar. Then replace the silicone cap on the cartridge. The paint is now contained and ready for storage.
The process is a little messy, but gets less so with practice.
Step 4: Related Things
I also made some non-stick mixing containers for paint out of silicone. If you don't wash them and the paint dries, it just peels right out.