Stenciling is a "dry brush technique". Usually it requires really tiny amount of paint. A set of different acrylic colors in small tube packings is very handy paint supply for different projects. However we face a problem when opening/ closing tube cups several times: the paint remains dry getting tube cups stuck, hard to open and close, etc.
Here is a solution: let's make a small hole with crafting knife, pouring the necessary small color amount through the hole. I have even learned to work the paint into stencil brush directly on the surface of tube and do not need any extra pads when working with small stenciling details. Fast and efficient!
Close the hole with a piece of tape when you are done. Drying paint remains will not be technically disturbing in this case, you can always open the hole with your crafts knife.