Game Of Thrones: Is Gendry Really Cersei Lannister's Son?
The Case For...
5. Black & Yellow Hair
The biggest and most commonly cited evidence for this theory comes from way back in Season 1, when we get to hear Cersei discuss the son she lost, and Gendry talk about the mother he never knew.
In Episode 2, The Kingsroad, Cersei tells a stricken Catelyn that she lost her firstborn child, referring to him as "a black haired beauty". Given all of Cersei's other children have her hair colour, it's at the very least true this child was Robert's rather than Jaime's. Gendry, meanwhile, has the black hair of the Baratheons.
That alone isn't much, but it's twinned with a similar sort of comment from Gendry in that same season, as he mentions to Ned what he told Jon Arryn about his mother, which is that she had yellow hair and used to sing to him. Gendry matches the description of her son, and Cersei fits with his mother. Game of Thrones doesn't often make such comments without importance, so why would Gendry, a bastard, talk about his mother with that key detail if there isn't more to it?