Come back to me, my dark army of Kirbys!
Anyway, the Saiya-jin have tails because they do...
Its just part of their anatomy.
Plus, their tail contains glands that, when contacted by radiation emitted by the light of a full moon, allows them to transform into gigantic apes..their Oozaru form
I read that little tidbit in the manga
Anyway....Sonic turns gold when he becomes Super Sonic for no special reason, most likely.
Maybe the creators were paying homage to DBZ....
Or maybe they just thought Sonic looked cool when they turned him gold and gave him more spikes
The world may never know...