okay, so say if someone has like a bad voice/tone or is really quiet, can they become a decent singer? through practice and training of course.

Yeah. I've known of "tone deaf" people becoming pretty good singers.

Yes

Having good timbre (tone color, in this case good/bad voice and the feel of it) and singing is different.
Singing well is very good
Having nice timbre is.. nice
Having both is dynamite.
Anyone can improve and a voice's tone is arbitrary.
I, for one, really do enjoy a theatrical kind of voice which isn't the case for a lot of people.