It might just be dirt. Did you try gently cleaning it off with the nail-file part of the clipper? (I'm assuming you are using a fine human-being kind of clipper, not a round dog-toenail clipper.)
I'm going to make some assumptions here. I assume you've been clipping your cats nails at least most of his life and that is why you're shocked at what appears to be a change in the color of his nail beds from what has been the norm for him.
There are many types of toenail infections, some serious, some indicative of other very serious diseases which a kitty can get. For these reasons I would take him to a veterinarian.
I hope your kitty is okay. You too.