As long as hair is strong enough for a color treatment, it can be colored. While most stylists recommend waiting a week between services to give the scalp a chance to recover, hair color services can be performed back to back and even on the same day.
To be deemed healthy enough to color, hair should not be overly faded or porous. If it feels spongy or if it won't hold color, it's too damaged to color. Additionally, when it is wet, the hair should stretch easily and bounce back without breaking. Placing color on a select strand of hair for a strand test indicates how the rest of the hair may hold up during the service.