Honestly, unless they are high-quality show bred white Plymouth Rocks, ear lobe color isn't going to mean much. It's shape and size that will mean more. Post a recent picture of them and I can give you my best guess.
As a side note, if you were meaning the egg at the bottom left of the picture you posted, it looked more like a light brown egg than a white egg to me. This is a plausible outcome from outcrossing white Rocks to white Leghorns in order to up production, which is what was done in some hatchery Plymouth Rock stock from what I understand.
And yes, sexlinks are a hybrid. Mutts are usually accidental crosses or crosses made on a whim. Hybrids are bred for a specific outcome, in this case being sexable upon hatch.
Pip, your knowledge base is impressive. You must read every bit of chicken litterateur available to you.
Good going kiddo!
! You take the egg turner out at day 18 and place them on the rack. When they hatch leave them in there to dry and wait for the majority hatch. They can survive 3 days without food I believe.
