If you suffered a recent below freezing weather episode as I did here in Colorado where we went from high 90s to low 20s in just a matter of hours, we could be looking at frostbite.
However, combs are sort of like those mood rings of the last century, they convey the current emotional state of a rooster and some hens with large combs. If you observe it, it can go back and forth from red to dark tips during the span of a day. It's nothing to be alarmed about.
However, as @Perris pointed out, it can point to a respiratory issue that prevents oxygen from reaching the outer extremities, turning the tips of the comb a dark, dusky dark purple. That can have serious implications if the chicken also has rails - crackly sounds as it breathes in and out. Such a chicken will behave lethargically due to the decreased O2 in its tissues. The cause is often tumors on the organs resulting in organ shutdown.
If this chicken is behaving normally, the discolored tips is nothing to be concerned about.