If you look closely, however, the secret may be out, as it's evident from the sweet snap that Archie has fair hair, with some royal watchers speculating that it appears to match his dad's hair colour - so he may be another red-headed royal.
It looks like Meghan was very much in charge of the recent post, as she captioned it to wish new dad Harry a happy Father's Day, which was celebrated in the UK on Sunday.
"Happy Father’s Day! And wishing a very special first Father’s Day to The Duke of Sussex! © SussexRoyal" she captioned the shot.
The Duke and Duchess of Cambridge's Kensington Palace Instagram account also paid tribute to fathers Prince William and Prince Charles.
"Happy Father’s Day!" a tribute post was captioned, alongside an adorable new photo of Prince William and Prince Louis, and a snap of Prince William with his dad, Prince Charles.