I played through the main game a bunch of times when it first came out but am only just now getting to the DLC, which prompted a fresh play through of the main game and noticed something when you first rescue Geppetto from his carriage (SPOILERS!!)
Carlo's eyes are brown compared to P's being blue, and thought it was an interesting decision for Geppetto to make, making a puppet in his son's image which is faithful in every way except for the eyes (the window to the soul etc), and just figured he couldn't face looking into his son's real eyes if he was just going to use P for spare parts/gathering ergo etc.
But when you first meet Geppetto in the main story his has very similar piercing blue eyes - he hates himself so much for not being able to protect his wife and son that in his own twisted way the cost of him getting his son back is looking into his own eyes reflected back at him by P. The least human eyes he could give this puppet, which would make the task of later killing and dissecting P bearable, are his own. This also made me think maybe Camille had brown eyes, and Carlo inherited them, so Geppetto couldn't bare the thought of his wife (metaphorically) seeing what he had become.
Crazy to still be noticing story details after 4-5 playthroughs!