Food coloring is good indefinitely and does not have an expiration date. Food coloring tends to hold on to its strength in color as well. Be sure the containers stay well-sealed.
The colors can get thicker or dried out over a long period of time. Just putting a few drops of very hot water in the bottles and giving them a good shake seems to solve that problem. When the bottle gets really low, adding some very hot water and shaking dilutes it enough to be able to get the last bit of color out of the bottle.
If one plans to airbrush, he should keep the little Americolor bottles to use for mixing and storing airbrush colors in different shades and for adding vodka, Everclear or extract easily.