There is no time limit on the shelf life of food colouring. Additionally, food colouring frequently maintains its colour intensity. Ensure that the containers remain tightly sealed.
Over time, the colours may become thicker or may dry out. That issue seems to be resolved by simply adding a few drops of extremely hot water to the bottles and vigorously shaking them.
When the colour in the bottle is nearly gone, adding some extremely hot water and shaking it makes it possible to extract the final trace of colour.
If someone intends to airbrush, they should keep the tiny Americolor bottles on hand to mix and store airbrush colours in various shades and to make it simple to add vodka, Everclear, or extract.