Food colouring has no expiration date and can be used continuously. Food colouring has a tendency to maintain its colour intensity. Make sure the containers are tightly sealed.
Over time, the colours may become thicker or more dried out. Adding a few drops of very hot water to the bottles and shaking them vigorously appears to alleviate the problem. When the bottle is completely empty, add some very hot water and shake it to dilute it sufficiently to get the last bit of colour out.
If you plan to airbrush, keep the little Americolor bottles on hand to mix and store several shades of airbrush colours, as well as to readily add vodka, Everclear, or extract.