Baby’s hair color too can be light at first and darken over time.
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That’s true. I see pictures of my dad when he was younger, and he was blond as a kid but his darker is darker now.
I wouldn’t worry about what baby Regina looked like, the baby had seconds of screen time. Though it would be interesting to see what her life was like growing up before The Stable Boy. Money/gold must have run out at some point.