And it only happens in the fight with Baby. Not any other times, like in the fight with Super 17. It is inconsistent. You're explaining away the inconsistency with your headcanon.>>186413787
Any official material is an official source. The Dragon Ball and Dragon Ball Z movies and OVAs are also official sources. The officially published spinoff manga like Heroes, Episode of Bardock, SD, Isekai Yamcha, are all official sources. The games and cardgames are official sources. Both the Dragon Ball Super manga and the Dragon Ball Super anime are official sources. Statues and figures are official sources. This is not the same as the dubious claims fans give to "canon" material, merely a reflection that it was something made in an official manner.
The official line going through this for nearly a decade now is that tails do not change color when transforming, as shown when Bardock, Raditz, Nappa, Cumber, the Saiyan hero in Ultimate Tenkaichi, the avatar Saiyans in Heroes, etc. etc. go Super Saiyan. The only notable exception is the ancient Shallot from the gacha game, who does not have a brown tail like most Saiyans in the first place, but a tail that is black like his hair, that goes gold.
GT does not have a consistent depiction of the matter. It is inconsistent, sometimes showing it gold, sometimes showing it brown. To give reason for why it is inconsistent is headcanon, because the series never explains it properly.