They'll be fine. Samples do eventually oxidize and dry out but that takes months. The factory puts the hair sample under a microscope to determine color and tones in the hair. They then record the color number in your record so even though it changes over time they have the original color on file. Of course if you've got gray hair coming in it's a good idea to increase the percentage of gray (or send a new sample) as time 'does it's thing.'
Regards,
John