Most hair that is produced for hairpieces is originally black. It must be processed, bleached, colored, and waved before it is used in hairpieces. The very light colors require lots of bleaching to take them from jet black all the way to white. So much bleaching weakens the hair. If you have a very light hair color, I recommend using synthetic fiber.
Gray hair is no problem.
You should, however, be very generous with your clippings. Some people intentionally put a little less gray into their hairpiece than they have naturally. Gray hairs must be synthetic, even if the basic piece is human hair overall. Every now and then we make a hairpiece that perfectly matches the samples provided, yet the customer is not happy with the color. The usual problem is that the color samples they send is not the color the customer remembers his hair.