I included this tip on a build report recently and it raised a little interest so thought it might be worth a heads-up here.
When I stay in hotels I tend to leave with some of the empty shampoo bottles for modelling use.
The best I have found are 30ml bottles as below - these from The Scottish Fine Soaps Company! Once rinsed out completely I use them to mix and thin paints for the airbrush. They are fully airtight and mixed paints last for weeks or months in them . It means I can mix a colour but keep plenty back for resprays or touch up. (When the sediment settles it's fine for brush applied touch up, or shake again for airbrush cup.)
The picture shows an original bottle with soap and two of the colours I mixed for a French 5 camo scheme, using Tamiya acrylics. However for my automobile models I also use them to mix Humbrol enamels with Mr Levelling Thinner, and also to decant Halfords rattle can paint through a plastic straw. In the latter case it takes a long time for the rattle can paint to gas-out and the lids need to be left loose otherwise the bottles blow up alarmingly (fortunately never had an explosion!!!)
You might think I am encouraging you to purloin lots of such bottles from the maids' trolleys in hotel corridors but I, of course, could not possibly condone that!
If it helps, all to the good.
Sandy