It’s not so much that their eyes change, but more so that it takes some time for their eyes and the colors of their eyes to “mature”, as they say. About three weeks into their life, puppies start to gradually get their final eye color.
Most dogs have dark brown eyes, and you'll be able to see some chocolate-colored spots starting to appear around this time. However, it happens slowly and isn't something that happens overnight. Often, it takes puppies nine to 12 weeks for their eye colors to stay and become permanent. Even at 16 weeks old, the permanent eye color may still be changing.